Deprecate system flag for registration, use cache instead

[Geod24]

The system flag likely predates the global 'cache' option. Logic dictates that 'dub add-path --cache=system' should behave the same as 'dub add-path --system' but that is not the case. However the PackageManager already has the right API in place, so it is just a matter of deprecating the layers in front.

[github-actions]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 11

Total warnings: 0

Build statistics:

 statistics (-before, +after)
 executable size=5375832 bin/dub
-rough build time=63s
+rough build time=61s

Full build output

DUB version 1.35.1, built on Jan  6 2024
LDC - the LLVM D compiler (1.36.0):
  based on DMD v2.106.1 and LLVM 17.0.6
  built with LDC - the LLVM D compiler (1.36.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: znver3
  http://dlang.org - http://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
   Upgrading project in /home/runner/work/dub/dub/
    Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/ldc2 for x86_64.
    Building dub 1.36.0-beta.1+commit.30.ga38e67ff: building configuration [application]
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/algorithm/iteration.d(475,42): Deprecation: function `dub.dub.getRegistryPackageSupplier` is deprecated - This function wasn't intended for public use - open an issue with Dub if you need it
source/dub/dub.d(67,57):        instantiated from here: `map!(immutable(string)[])`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/algorithm/iteration.d(582,19): Deprecation: function `dub.dub.getRegistryPackageSupplier` is deprecated - This function wasn't intended for public use - open an issue with Dub if you need it
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/algorithm/iteration.d(479,16):        instantiated from here: `MapResult!(getRegistryPackageSupplier, immutable(string)[])`
source/dub/dub.d(67,57):        instantiated from here: `map!(immutable(string)[])`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/algorithm/iteration.d(594,23): Deprecation: function `dub.dub.getRegistryPackageSupplier` is deprecated - This function wasn't intended for public use - open an issue with Dub if you need it
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/algorithm/iteration.d(479,16):        instantiated from here: `MapResult!(getRegistryPackageSupplier, immutable(string)[])`
source/dub/dub.d(67,57):        instantiated from here: `map!(immutable(string)[])`
source/dub/internal/configy/Exceptions.d(248,27): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(250,27): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(283,27): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(286,27): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(323,31): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(325,31): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(332,31): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
source/dub/internal/configy/Exceptions.d(335,31): Deprecation: `@safe` function `formatMessage` calling `formattedWrite`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/write.d(537,34):        which calls `std.format.spec.FormatSpec!char.FormatSpec.writeUpToNextSpec!(void delegate(in char[]) @safe).writeUpToNextSpec`
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        which wouldn't be `@safe` because of:
/opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/../import/std/format/spec.d(258,33):        scope variable `this` assigned to non-scope parameter `e` calling `put`
     Linking dub
STAT:statistics (-before, +after)
STAT:executable size=5375832 bin/dub
STAT:rough build time=61s