Print a better exception message and handle this in more cases rather
than producing confusing errors when it’s unset.

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.

Since #292, HOMEBREW_CACHE was moved to a per-user directory. This makes
it unsuitable to store global lock files on multiple users environment.

Therefore, introducing a global lock directory `/Library/Lock.d` to
store lock files from formula lockers as well as `brew update`.

* cleanup: accept cache as an argument.

* config: move default HOMEBREW_CACHE to ~/Library.

* brew.1: document new default Homebrew cache.

* update-report: migrate legacy Homebrew cache.

`Library/ENV` like `Library/Homebrew` is part of Homebrew basecode. It
should be able to be accessed during the `brew tests`.

By adding HOMEBREW_ENV_PATH variable, we allow test suit to locate these
codes.

The default HOMEBREW_LIBRARY_PATH should be
  /usr/local/Library/Homebrew
not
  /usr/local/Library/Homebrew/Homebrew

Closes Homebrew/homebrew#49384.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Currently HOMEBREW_BREW_FILE is a String, while other of HOMEBREW_*
variables are all Pathname. This commit unifies them all as Pathname,
so it will not cause any confusion.

Closes Homebrew/homebrew#48872.

Signed-off-by: Xu Cheng <xucheng@me.com>

Not sure why this is happening (beyond the Chef cookbook stupidly
deciding to not call through `bin/brew`) but fail and print a scary
looking error to hope to point people in the right direction.

Closes Homebrew/homebrew#48261.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Closes Homebrew/homebrew#42354.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Fixes Homebrew/homebrew#40752.

This reverts commit b95f27ce9f956c56d080b6dc29ba8efb841c6690.

This allows global.rb to be safely loaded in the test environment.