Move some stuff formerly in `Library/ENV` around:
- Move `Library/ENV/$XCODE_VERSION` to `Library/Homebrew/env/super` as they are
all superenv wrappers and all symlinks to the same version. We never needed
the "separate shims for separate versions" functionality and it just adds
confusion.
- Move `Library/ENV/pkgconfig` to `Library/Homebrew/env/pkgconfig` to get more
things under `Library/Homebrew`
- Move `Library/ENV/scm` to `Library/scm` as these wrappers are not actually
used by or related to superenv (or stdenv) in any way.
In 0d189fae57
we completely removed `effective_sysroot`, which consequently left all the
`effective_sysroot`/usr/include calls pointing at `/usr/include`. This is wildly
problematic on systems where the Command Line Tools aren't installed because
`/usr/include` is not a default-created folder prior to CLT installation.
I'm unsure if `effective_sysroot` should still be mentioned in Library/Homebrew/extend/ENV/super.rb
at all. If it can be deleted, feel free to do that without waiting for me to review.
This seems to fix:
* https://github.com/Homebrew/homebrew-core/issues/2991
* https://github.com/Homebrew/homebrew-core/issues/2986
* https://github.com/Homebrew/homebrew-core/issues/2962
