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`.

<list>
    <customer>
        <id>23</id>
        <name>Kurt Meier</name>
    </customer>
    <customer>
        <id>42</id>
        <name>Lieschen Schmidt</name>
    </customer>
</list>