pkgset: Add SRPMs to whitelist
    
    When cherry picking packages from Koji tag, we need to make sure that
    for every binary package we always have a corresponding source package.
    Even if it does not go into the compose, we need it to get values for
    Exclusive or Exclude Arch tags.
    
    This means we need to process the binary packages first and only then
    look at source ones. Instead of sorting a potentially very long list,
    let's just iterate twice.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>