Remove comps groups from purely modular variants
    
    The comps source should not return all groups when there are only
    modules defined. This fixes part of the problem: non-modular packages
    will not go in by default.
    
    The second part is the comps file in the created repository. It will be
    filtered to not contain any groups (because packages from there will not
    be in the repo).
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>