Migrate all Oscar related Julia packages to use JLLs

[fingolfin]

This brings several advantages.

• it is the way forward from the Julia community
• as such, we benefit from work others put into building standard libraries like GMP, readline, MPFR, etc., and others benefit from the work we invest into packaging things like Flint, normaliz, NTL, etc.
• precompiled binaries are only downloaded once and cached as "artifacts"
• it ensures that we only have one copy of each used library (right now, we install e.g. Singular twice, once inside Singular.jl, once inside Polymake.jl)
• a uniform way to override these binary artifacts so that we can use e.g. a development version of Flint or Singular or GAP or ... while working on our Julia packages

The only real drawback seems to be that JLLs require Julia >= 1.3 but that's already the case for e.g. GAP.jl so not a major loss (with perhaps the exception of LoadFlint.jl, see below). Once the next Julia LTS (1.6?) is out, I hope we can just move to that and all will be well.

Here is a list of components that might benefit from this and what needs to be / what the status is:

• GAP.jl:
  • work in progress at Switch to using JLLs GAP.jl#421, @fingolfin should take care of it
  • first version of a GAP_jll (still not fully ready): New recipe: GAP JuliaPackaging/Yggdrasil#1342
• Singular.jl:
  • New Recipe: Singular 4.1.3 JuliaPackaging/Yggdrasil#1163 adds a Singular_jll, waiting for it to be merged; however this is currently built without NTL
  • we need to take care of the missing NTL in one of two ways:
    1. improve the NTL builder to cross compile to at least macOS (ideally more) and then adjust the Singular builder to use that
    2. alternatively, the Flint support in Singular should be improved by @hannes14 and others to make it possible to drop NTL completely
  • Singular.jl needs to be adapted to use Singular_jll, see Switch Singular.jl from SingularBuild to Singular_jll; create and use `libsingular_jll.jl Singular.jl#254 and Use Singular_jll Singular.jl#256
• LoadFlint.jl: work in progress at WIP: Pkg Artifacts with legacy mode LoadFlint.jl#4 (with the extract complication that this tries to retain compatibility with Julia 1.0-1.2
• Nemo.jl:
  • requires Arb, Antic, Flint (via LoadFlint.jl for now), for all of these we have JLLs (thanks to @thofma and @benlorenz)
  • needs a PR which actually makes it use those JLLs: Update to LoadFlint 2.0 and rehaul build system Nemocas/Nemo.jl#845
• Polymake.jl: that one is probably the toughest by far but luckily some really tough people are working on it :-) so it is not completely hopeless... indeed I think @benlorenz created several of the JLLs listed below...
  • requirements for which there are already JLLs (although I obviously did not check if the respective JLLs are already "good enough"): MPFR, GMP, ncurses, readline, boost, ppl, lrslib, bliss, normaliz, ninja, flint, NTL (with the same caveat as listed above), Singular
  • missing JLLs: perl: [Perl] initial builder for 5.30.3 for selected platforms JuliaPackaging/Yggdrasil#1293
  • Polymake_jll: [polymake] builder for x86_64 linux and mac os JuliaPackaging/Yggdrasil#1368
  • libpolymake_julia_jll: [libpolymake_julia] new recipe JuliaPackaging/Yggdrasil#1423
  • final PR adding this all to Polymake.jl: split libpolymake_julia and use artifacts Polymake.jl#308
  • tag and register a new 0.5.0 release

In addition, we should carefully document somewhere (possibly separately in each of the packages listed above) how to setup those artifact overrides so that people who work on GAP/Singular/Flint/Polymake/etc. can use dev versions of those.

[fingolfin]

This is mostly done. There is still more we can do, in particular, GAP.jl still compiles GAP packages (I plan to work on that). But the overall goal has been reached.