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.

This reverts commit dba1958bd79c1c9d18f215dfc2b806ea62edd1c8.

This reverts commit 252c701c59227c385ef6178fe99523cca8c843bb.

Taps installed prior to running the test suite are not visible to the
test suite as most Homebrew paths are redefined as to not mess up the
local installation.

All of these taps use Homebrew internal APIs (or will shortly) and we
autoinstall them all from `brew $CMD`. We should adjust our CI to ensure
that we never accidentally break these taps when making changes to core
code so that these taps can rely more on this core code rather than
having to e.g. vendor equivalent code that never changes on our end.

If the environment variable HOMEBREW_TEST_GENERIC_OS is set ensure that
neither Mac nor Linux-specific code is loaded. This allows easier
testing of cross-platform code on OS X and will make it easier to port
Homebrew to platforms other than OS X and Linux.

Global namespaces are good to avoid when possible.

This version is never wanted at this point and it will help Homebrew
Cask deal with the annoying errors that result from having this version
still around (some which I've already help users debug).

Use the `reason` attribute to be able to handle `UsageError` subclasses
more uniformly and simplify logic in `brew.rb` to handle them together.