Basetools: Include PCD declarations from Library Instance

The patch "[PATCH v3 1/2] StandaloneMmPkg: Make StandaloneMmCpu driver
architecture independent" (https://edk2.groups.io/g/devel/message/109178)
removed ArmPkg/ArmPkg.dec from the Packages section in the
INF file: StandaloneMmPkg/Drivers/StandaloneMmCpu/StandaloneMmCpu.inf

This change was done as part of making the StandaloneMmCpu driver
architecture independent.

Although this change is correct, it results in a side effect
here some platforms that utilise PCDs declared in ArmPkg.dec are
no longer declared.

An example of this issue can be seen when building
edk2-platforms/Platform/ARM/SgiPkg/PlatformStandaloneMm.dsc

$ build -a AARCH64 -t GCC -p Platform/ARM/SgiPkg/PlatformStandaloneMm.dsc
build.py...
/mnt/source/edk2-platforms/Platform/ARM/SgiPkg/PlatformStandaloneMm.fdf(23):
  error F001: PCD (gArmTokenSpaceGuid.PcdFdBaseAddress) used in
  FDF is not declared in DEC files.

As seen above, removing ArmPkg.dec from the Packages section in the
StandAloneMmCpu Driver Inf file triggers build failure.
Although, ArmPkg.dec is included in other Library Instances,
the build system does not include the declarations from
.dec files defined in Library instances.

The build system only includes the PCD declarations from DEC files
that are specified in INF files for Modules (components).

Therefore, extend the build system to include the Packages from
Library Instances so that the PCD declarations from the respective package
DEC files are included.

This patch can be seen on
    https://github.com/LeviYeoReum/edk2/tree/levi/2848_dec_check_on_library

Signed-off-by: levi.yun <[email protected]>
Tested-by: Pierre Gondois <[email protected]>
Reviewed-by: Rebecca Cran <[email protected]>
Reviewed-by: Sami Mujawar <[email protected]>

BaseTools/Source/Python/AutoGen/WorkspaceAutoGen.py

@@ -439,6 +439,10 @@ def PkgSet(self):
             PkgSet = set()
             for mb in [self.BuildDatabase[m, Arch, self.BuildTarget, self.ToolChain] for m in Platform.Modules]:
                 PkgSet.update(mb.Packages)
+
+            for lb in [self.BuildDatabase[l, Arch, self.BuildTarget, self.ToolChain] for l in Platform.LibraryInstances]:
+                PkgSet.update(lb.Packages)
+
             for Inf in ModuleList:
                 ModuleFile = PathClass(NormPath(Inf), GlobalData.gWorkspace, Arch)
                 if ModuleFile in Platform.Modules:
@@ -968,4 +972,3 @@ def CreateCodeFile(self, CreateDepsCodeFile=False):
     #
     def CreateAsBuiltInf(self):
         return
-