critpath.py: revise and fix URL arguments
    
    This completely revises the logic for determining repo URLs and
    handling alternative arguments. The existing code had outdated
    hacks for running on nightly composes which haven't worked since
    at least 2017. Throw those out, and refactor so you can pass
    alternate URLs to the top-level fedora/linux and fedora-secondary
    directories for working with repos (e.g. for using a different
    mirror or a local filesystem if you have the repos there), *or*
    you can pass a URL to the /compose directory of a non-split
    Pungi compose, which overrides the other two settings if set.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>