Move the content of swh/objstorage/__init__.py in swh/objstorage/factory.py
This is needed to "empty" the swh.objstorage's module from any non-subpackage objects, so we can implement the swh.objstorage.replayer outside the swh.objstorage's repo, in a dedicated repo/project.
Migrated from D3062 (view on Phabricator)