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.

Fixes Homebrew/homebrew#40656.

Closes Homebrew/homebrew#16063.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Closes Homebrew/homebrew#15961.

10.8 no longer ships four .pc files which were included in previous
versions, though it does still include the libraries which they pointed
to. This commit provides copies of the .pc files for all of these, and
prepends Library/Homebrew/pkgconfig to PKG_CONFIG_PATH on 10.8.

The .pc files in question are: lib curl, lubexslt, libxml-2.0, libxslt

Fixes Homebrew/homebrew#13673.
Fixes Homebrew/homebrew#13627.
Fixes Homebrew/homebrew#13652.
Fixes Homebrew/homebrew#13482.
Closes Homebrew/homebrew#13572.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>