revert moving "creating packages" from Pkg.jl (#53509)
As mentioned in
https://github.com/JuliaLang/julia/pull/52102#issuecomment-1845523326,
having this information be moved from Pkg to Base creates a quite
uncomfortable split between information about the package manager
between the Pkg docs and Base.
Pkg PR: https://github.com/JuliaLang/Pkg.jl/pull/3818
