Fix Pkg.jl invalidations

Move Pkg.BinaryPlatforms to Base.BinaryPlatforms.PkgCompat
Move UnstableIO to Base

Corresponding Julia PR (merge this first): https://github.com/JuliaLang/julia/pull/52249

Fix #3702 . No more invalidations.

julia> using SnoopCompileCore

julia> invalidations = @snoopr begin
           using Pkg
           Pkg.activate()
       end
  Activating project at `~/.julia/environments/v1.11`
2-element Vector{Any}:
 MethodInstance for Base.typeinfo_implicit(::Any)
 "invalidate_mt_cache"

julia> using SnoopCompile
Precompiling SnoopCompile
  4 dependencies successfully precompiled in 78 seconds. 58 already precompiled.
[ Info: Precompiling SnoopCompile [aa65fe97-06da-5843-b5b1-d5d13cad87d2]

julia> trees = invalidation_trees(invalidations)
SnoopCompile.MethodInvalidations[]

JuliaLang / Pkg.jl

Fix Pkg.jl invalidations #3712