rework the cli db tooling for winery

These changes are necessary to make the objstorage winery backend fit in here.

The issue was in winery, the db settings are not at the top level of the
'winery' backend configuration, but under the "database" subsection,
which was an unsupported case.

These changes make it work, but for simplification purpose, we drop the
support for 'full module path' specification of the target backend to
initialize. Namely, one cannot use the pattern:

  # this will not work any more
  swh db init -d DBCNX swh.storage.backend.masking

but stick to the entry-point pattern:

  swh db init -d DBCNX storage:masking

Or use the config file based approach.

Also comes with a few cleanup revisions:

• remove --module-config-key option
• remove support for xxx_db configuration entries
• Move import_swhmodule helper function in swh.core.config
• db: Drop BW compat for non-entrypoint aware swh packages