Support Drupal core repositories.

This provides support for Drupal core repositories such as https://github.com/drupal/drupal or https://github.com/tstoeckler/drupal-core

See http://cgit.drupalcode.org/drupal/tree/composer.json#n4

As Drupal (sadly) depends on a very particular file structure this is required in order to setup a full Drupal install via Composer.