Add a derivative discovery decorator that uses services as plugin derivers.

Good morning!

I briefly discussed this with neclimdul and EclipseGc on IRC yesterday... Currently we are registering plugin managers with the DIC which might use derivative discovery - In order to retrieve a list of existing entity types in a plugin deriver we would now have to use entity_get_info() or (in the future) drupal_container()->get('plugin.manager.entity'). I would very much prefer to inject stuff into my plugin derivers directly. Does it make sense to add a discovery decorator to \Core which would refer to plugin derivers as service ids instead of FQCNs?