Refactor Pkg.BinaryPlatforms to avoid invalidations, keep types

[mkitti]

This is my third attempt to address invalidations caused by Pkg.BinaryPlatforms.

The prior attempts were

• Move Pkg.BinaryPlatforms to Base Fix Pkg.BinaryPlatforms invalidations by moving module to Base julia#52249
• Refactor Pkg.BinaryPlatforms by eliminating the AbstractPlatform subtypes Refactor Pkg.BinaryPlatforms compat, fix invalidations #3736

In this pull request, I introduce a new Pkg.BinaryPlatforms.AbstractPlatform type
that is distinct from Base.BinaryPlatforms.AbstractPlatform. The types here, Linux,
Windows, MacOS, and FreeBSD subtype Pkg.BinaryPlatforms.AbstractPlatform.

Because of this, these types no longer invalidate any Base methods as detailed in
#3702

Fixes #3702 (in part).

[mkitti]

After this pull request, the only remaining invalidations relate to UnstableIO.

julia> using SnoopCompileCore

julia> invalidations = @snoopr begin
           using Pkg
           Pkg.activate(".")
       end;
  Activating project at `~/src/Pkg.jl`

julia> using SnoopCompile
[ Info: Precompiling SnoopCompile [aa65fe97-06da-5843-b5b1-d5d13cad87d2]

julia> invalidation_trees(invalidations)
1-element Vector{SnoopCompile.MethodInvalidations}:
 inserting print(io::Pkg.UnstableIO, arg::Union{SubString{String}, String}) @ Pkg ~/src/Pkg.jl/src/Pkg.jl:49 invalidated:
   backedges: 1: superseding print(xs...) @ Base coreio.jl:3 with MethodInstance for print(::Any, ::String) (2 children)
              2: superseding print(io::IO, s::Union{SubString{String}, String}) @ Base strings/io.jl:250 with MethodInstance for print(::IO, ::String) (362 children)
   1 mt_cache

[staticfloat]

Yes, this is my favorite attempt so far!

[mkitti]

The main breaking change is that Linux is no longer a subtype of Base.BinaryPlatforms.AbstractPlatform, henceforth referred to as BBP.AbstractPlatform.

BBP.AbstractPlatform was meant to ease the implementation of Pkg.BinaryPlatforms. Now BBP.AbstractPlatform has no purpose if this is merged. BBP.Platform would be its only subtype.

A complementary change on the Base side would be to define methods for BBP.Platform rather than BBP.AbstractPlatform. After doing so, BBP.AbstractPlatform may be free again to act as a common abstract parent type for BBP.Platform and the OS-specific types.