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)

swh/objstorage/api/server.py

@@ -19,7 +19,7 @@ from swh.core.api.asynchronous import (
 from swh.core.api.serializers import msgpack_loads, SWHJSONDecoder
 
 from swh.model import hashutil
-from swh.objstorage import get_objstorage
+from swh.objstorage.factory import get_objstorage
 from swh.objstorage.objstorage import DEFAULT_LIMIT
 from swh.objstorage.exc import Error, ObjNotFoundError
 from swh.core.statsd import statsd