Windows installer should respect existing copies of Python better

Registered by rdb

We need to have the Panda3D installer on Windows have an option to, instead of just plain installing its own version of python, put the panda.pth files in the existing python site-packages dir.
We'd need to implement that here, I think:
http://panda3d.cvs.sourceforge.net/viewvc/panda3d/direct/src/directscripts/packpanda.nsi?view=markup

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