**API**: Refactor inheritance to be more generic
    
    The big change here is to introduce a concept of productmd-ness.
    The compose result class is really a productmd compose class,
    properly considered (at least, it's for composes with productmd
    style compose IDs). We avoid use of fedfind in parsing the ID,
    to allow for a future commit splitting Fedora-specific classes
    into a separate module.
    
    We introduce a class for generic productmd-type images, which
    requires only the image dict and compose ID, and make the Fedora
    image class a subclass of that which provides the image dict
    to it.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>