comps-wrapper: Port to libcomps
    
    Instead of replacing yum.comps with an something from DNF, we can go
    directly to libcomps. DNF does not have the equivalent functionality
    (particularly it's impossible to load comps from file directly).
    
    We would have depended on libcomps anyway transitively, so this is not a
    big deal.
    
    Fixes: #587
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>