split-files: move static filelist config into split-files.py
    
    The idea was that it's nicer to keep that config in .spec where it's subject
    to syntax highlighting. split-files.py was supposed to a stand-alone program.
    But in practice this split is confusing, because file rules are listed in two
    places and we need to modify split-files.py quite often. This will be easier if
    everything is in one file.
    
    [skip changelog]