Convert yml2 into a Python package.

* Rename yml2.py -> yml2/__init__.py
* Rename backend.py -> yml2/backend.py
* Rename pyPEG.py -> yml2/pyPEG.py
* Use relative imports where appropriate.
* Change imports in yml2c, yml2proc.
* Update make target `clean`.

liste {
    adresse {
        name "Meier";
        strasse "Uhlmannstrasse 42";
        ort "Laupheim";
    }
    adresse {
        name "Schmidt";
        strasse "Haupstrasse 23";
        ort "Bad Waldsee";
    }
}