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`.
decl manpage(xmlns="http://docbook.org/ns/docbook", version="5.0", *title) alias article { title *title; content; };