core.cli: Add auto-register loader subcommand mechanism
This allows to define loader subcommand to the current swh command mechanism.
Note: I did not find a suitable way to test this yet. I tried to test via patching
the pkg_resources.iter_entry_points
but that fails one way or another.
Related to T2134
Test Plan
tox
Migrated from D2388 (view on Phabricator)