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`.
include yslt.yml2
tstylesheet {
template "/list/*"
| INSERT INTO «name()» (`apply "*", 0, mode="names"`) VALUES (`apply "*", mode="values"`);
template "*", mode="names" > «name()»`if "position()!=last()" ", "`
template "*", mode="values" > '«.»'`if "position()!=last()" ", "`
}