[python] don't include site-packages in python path

This is very error prone as add-on may 'silently work' if required dependence are installed system-wide. It's basically impossible to test if your add-on really works without a clean python install.

I have no idea why site-packages was [added](https://github.com/xbmc/xbmc/commit/c4fd22dd326af8a82c50d6f7d0df85a6518aa14b) in the first place. May completely break something.


@stefansaraev It's supposed to be for linux only as others don't need any loading script. I'm stuck at how add it to build script so it's moved here on build instead.
@MartijnKaijser exactly what folder is it moved into? It should be `script.module.pil/lib/PIL/PIL.py` etc correct?
I can't test on win/osx/android.