Use correct namespace for SIGs during validation
    
    When validating the spelling of the package during cloning, decide based
    on the program name whether to use the default namespace of CentOS RPMs,
    or the namespace dedicated to CentOS SIGs.
    
    Note that there is no reasonable way to deduce the specific SIG, so it
    is required to give absolute path, e.g.,`‹SIG›/rpms/‹package›` when
    cloning.
    
    Signed-off-by: Matej Focko <mfocko@redhat.com>