ENV: move to new paths. (#507)

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.