Add a little script to check for missing packages in comps
    
    It would be nice to make 'package is listed in group, but does
    not exist' a fatal error (at least optionally) for dnf etc, but
    we can't really do that at present because it's *not* been a
    fatal error for so long that our comps have tons of stale
    entries for packages that no longer exist, and also entries that
    aren't properly archified for packages which only exist on some
    arches. This script pokes through the comps file for current
    Rawhide and identifies all (I hope) pkgreqs which specify a
    package that is not actually available in current Rawhide, per
    arch.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>