cli: add support for aliases in click command groups
this allows to rename (sub)commands while keeping the old name working, albeit not listed in help messages.
Say one wants to rename: swh scheduler api-server as swh scheduler rpc-serve
but keep the former version usable.
Also fix logging config in main's swh command we want the root logger configured there, not the cli one.
Related to #1671.
Migrated from D1479 (view on Phabricator)