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" {
        | CREATE TABLE «name(*)» (
        
        apply "*[1]/*";

        | );
    }

    template "*" {
        | «name()» VARCHAR`if "position()!=last()" ", "`
    }
}