Move drush from main homebrew to homebrew-php

 * Moved drush from main homebrew to homebrew-php.

   The drush formula now requires a build dependency on composer which is
   located in homebrew-php.

 * Drush HEAD build depends on composer.
    
   The HEAD version of drush has a build dependency on composer.

See Homebrew/homebrew#31696 and Homebrew/homebrew#30596 for issues regarding not having the formula in the main Homebrew repository anymore.

Ping; @mikemcquaid.
I actually want them to point to `HOMEBREW_PREFIX`.

They are used to point to locations where other formulas can install commands for drush and locations for config files that should persist when upgrading this formula - so it has to be outside of `prefix`.