critpath.py: prepare for critical-path-base group split
    
    See https://pagure.io/fedora-comps/pull-request/797 - we need the
    script to be ready for that change before we land it, or else
    the packages in the new groups will stop being considered to be
    in the critical path.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>