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`.
110 DATA "Meier", "Uhlmannstrasse 42", "Laupheim"
120 DATA "Schmidt", "Haupstrasse 23", "Bad Waldsee"
1000 FOR I=1 TO 2 STEP 1
1010 READ NAME$, STREET$, VILLAGE$
1020 PRINT "INSERT INTO address (name, street, village) VALUES ('" + NAME$ + "', '" + STREET$ + "', '" + VILLAGE$ + "');"
1030 NEXT I