Use './builtin' as the built-in module name instead of
None. Clarify the typing that this is now always a string.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210201193747.2169670-9-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>