Fix packages getting erased on failed update with dnf (RhBug:1620275)
    
    When adding update elements, we set the erase element to depend on the
    install element, but if an API user adds the same erasure manually
    after adding the update, we know its a duplicate erasure and filter
    it out, BUT we zero out the dependent element in the process. And
    if installing the update now fails, we end up removing the whole package
    due to that missing dependent element.
    
    This never happens with rpm itself so we can't easily test it, but is
    100% reproducable with dnf (at least dnf 3-4). Apparently it adds all
    erasures by itself (which is kind of understandable I guess, perhaps
    we should better allow this in the API)