Convert AbstractPlatform to Platform

[mkitti]

This pull request hardens Pkg.jl from invalidations involving AbstractPlatform. When AbstractPlatform is used as the type of a keyword argument, Julia may not
specialize on the type.

This pull request inserts convert(Platform, platform)::Platform to all functions that take platform::AbstractPlatform as a keyword argument.

[mkitti]

Before this pull request:

julia> using SnoopCompileCore, Pkg

julia> invalidations = @snoopr using BinaryBuilderBase;

julia> using SnoopCompile

julia> trees = invalidation_trees(invalidations)
3-element Vector{SnoopCompile.MethodInvalidations}:
 inserting joinpath(prefix::Prefix, args...) @ BinaryBuilderBase ~/.julia/packages/BinaryBuilderBase/tGUXK/src/Prefix.jl:68 invalidated:
   mt_backedges: 1: signature Tuple{typeof(joinpath), Any, String} triggered MethodInstance for Artifacts.jointail(::Any, ::String) (0 children)

 inserting repr(p::FileProduct) @ BinaryBuilderBase ~/.julia/packages/BinaryBuilderBase/tGUXK/src/Products.jl:463 invalidated:
   backedges: 1: superseding repr(x; context) @ Base strings/io.jl:286 with MethodInstance for repr(::Any) (43 children)

 inserting tags(p::AnyPlatform) @ BinaryBuilderBase ~/.julia/packages/BinaryBuilderBase/tGUXK/src/Platforms.jl:14 invalidated:
   mt_backedges:  1: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.triplet(::Base.BinaryPlatforms.AbstractPlatform) (0 children)
                  2: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.libgfortran_version(::Base.BinaryPlatforms.AbstractPlatform) (1 children)
                  3: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.cxxstring_abi(::Base.BinaryPlatforms.AbstractPlatform) (1 children)
                  4: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.libstdcxx_version(::Base.BinaryPlatforms.AbstractPlatform) (1 children)
                  5: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.os_version(::Base.BinaryPlatforms.AbstractPlatform) (2 children)
                  6: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.call_abi(::Base.BinaryPlatforms.AbstractPlatform) (2 children)
                  7: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.arch(::Base.BinaryPlatforms.AbstractPlatform) (2 children)
                  8: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.os(::Base.BinaryPlatforms.AbstractPlatform) (2 children)
                  9: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.var"#Platform#8"(::Base.Pairs{Symbol, Union{}, Tuple{}, @NamedTuple{}}, ::Type{Platform}, ::Base.BinaryPlatforms.AbstractPlatform) (5 children)
                 10: signature Tuple{typeof(Base.BinaryPlatforms.tags), Base.BinaryPlatforms.AbstractPlatform} triggered MethodInstance for Base.BinaryPlatforms.libc(::Base.BinaryPlatforms.AbstractPlatform) (60 children)

After this pull request:

julia> using SnoopCompileCore, Pkg

julia> invalidations = @snoopr using BinaryBuilderBase;

julia> using SnoopCompile

julia> trees = invalidation_trees(invalidations)
2-element Vector{SnoopCompile.MethodInvalidations}:
 inserting joinpath(prefix::Prefix, args...) @ BinaryBuilderBase ~/.julia/packages/BinaryBuilderBase/tGUXK/src/Prefix.jl:68 invalidated:
   mt_backedges: 1: signature Tuple{typeof(joinpath), Any, String} triggered MethodInstance for Artifacts.jointail(::Any, ::String) (0 children)

 inserting repr(p::FileProduct) @ BinaryBuilderBase ~/.julia/packages/BinaryBuilderBase/tGUXK/src/Products.jl:463 invalidated:
   backedges: 1: superseding repr(x; context) @ Base strings/io.jl:286 with MethodInstance for repr(::Any) (43 children)

[mkitti]

I'm highlighting this change.

Here we convert most of the AbstractPlatform subtypes to Platform by unwrapping them.

The one outlier is UnknownPlatform but there is another pull request to address that.