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.
Drush will look for a config file in `$ETC_PREFIX/drush/drushrc.php`.

To make the file persistent over upgrades it needs to be in (with `HOMEBREW_PREFIX`being `/usr/local`) `/usr/local/etc/drush/drushrc.php`.

If I used `prefix`it would end up being `/usr/local/opt/drush/etc/drush/drushrc.php` and that file would be nuked when the formula is upgrade.

It's the same story with `$SHARE_PREFIX`. Drush will look in `$SHARE_PREFIX/share/doc/drush/commands` for commands installed by other formulas (`drupal-code-sniffer.rb` does that).

They would also be nuked if installed inside `prefix`.

So, the reason for using `HOMEBREW_PREFIX` is to point outside `prefix` for stuff that should survive an upgrade.