cli: make swh-scheduler a subcommand of swh.core's main "swh" command
the old "swh-scheduler" still exists for now, and is mostly backward compatible but the --log-level option (which is defined in the "swh" main command group).
Unit tests have been adapted and should pass ok.
Depends on swh-core!57 (closed). Related to swh-core#1671.
Migrated from D1439 (view on Phabricator)