cli: add a generic main 'swh' command

This allows to declare click-based subcommands using the setuptools'
entry point "swh.cli.subcommands".

The main "swh" command group does only handle the "--log-level" option.

Also extract the db-init command in a dedicated submodule and ensure both

swh db-init

and

swh-db-init

work.

Related to T1671.