move list command to archiver.list_cmd

note: can't be named "archiver.list" due to collision with builtin type "list".

src/borg/archiver/__init__.py

@@ -97,7 +97,7 @@ from .delete import DeleteMixIn
 from .diff import DiffMixIn
 from .help import HelpMixIn
 from .keys import KeysMixIn
-from .list import ListMixIn
+from .list_cmd import ListMixIn
 from .locks import LocksMixIn
 from .mount import MountMixIn
 from .prune import PruneMixIn