In order to remove warnings about /apidoc/*.rst files being included
multiple times in toc when building full swh documentation, prefer to
include module indices only when building standalone package documentation.

Also include them the proper sphinx way.

Related to T4496

It enables to align that parameter name with the one used in
django.contrib.auth.

Migrate auth command group from swh-web-client package to the
swh-auth package.

Related to T3385