gather: nodeps should take packages from comps groups
    
    When gather_method is set to nodeps, we should not ignore the comps
    group that the method received. Instead it should find out which
    packages are in those groups and take them into the compose.
    
    In order for this to be of any reasonable use, the comps file needs to
    include all dependencies for the packages.
    
    Fixes: #653
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>