Deprecate made-up URL schemes

This deprecates URLs that begin with the following fake URL schemes:
  bzr://
  cvs://
  hg://
  fossil://

Despite their appearance, they are not actually accepted by the
corresponding tools. They exist only as hints to the download strategy
detection code, and are stripped from the URL before the URL is handed
off to the VCS program.

This is not documented, and may lead one to believe it is a valid URL
that can be used elsewhere, when it actually only works inside Homebrew.

We have a better mechanism for specifying the download strategy that is
straightforward and explicit:
  :using => :hg

Recommend that instead.