Add content_licenses to module metadata
    
    During the createrepo phase for Modular variants, this will now
    interrogate the repodata from the "work" repositories for the set of
    licenses in use by each of the RPMs in a module and add those to the
    metadata to be written out into the final repodata location.
    
    Merges: https://pagure.io/pungi/pull-request/968
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>