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`.

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include yslt.yml2

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stylesheet {

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    template "/list" html body table border=1 apply "address";

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    template "address" tr {

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        td value "name";

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        td value "street";

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        td value "city";

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    }

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}