frontend, backend: support modulemd v2
    
    Drop some of our modularity implementation details and move the code to
    the separate `modulemd_tools` library. As a side-effect, update the rest
    of our code from using `Modulemd 1.0` to `Modulemd 2.0` and therefore
    supporting both v1 and v2 modulemd YAML files.
    
    The `ModulemdGenerator` class underwent a notable simplification and
    most of its methods are now just calling `modulemd_tools.yaml.update`
    and therefore can be refactored even further but as a proof of concept,
    I am happy with the current implementation.