allows to declare worker tasks in a 'swh.workers' entry point. This later is expected to be a callable which returns a dict which 'tasks' key is
the module name of the celery tasks exported.
Some other 'hooks' could be provided as callable in this dictionnary,
typically db initialization stuff in listers could be handled that way.
Not sure what a decent 'API' for this shoudl look like for now, still