Update for extraction of ModuleInstaller class

The `ModuleInstaller` class has been extracted from the `ModuleHandler` class in order to isolate the `install()` and `uninstall()` methods (see [Drupal #2324055](https://www.drupal.org/node/2324055)).

This PR adapts the Drush environment functions `drush_module_install()` and `drush_module_uninstall()`.