Use modulemd.dump_all to create local repodata
    
    Both pungi and dnf use the new multidocument format, with the
    old one not really being supported by anything at this point.
    Let's be compatible.
    
    This change requires modulemd-1.2.0+.
    
    Signed-off-by: Petr Šabata <contyk@redhat.com>