Fixed 3rd-party package installation for namespace packages and overriding built-in libraries

Replacing sys.path.insert() with site.addsitedir() makes it possible to import namespace package dependencies.
If the goal is to turn this into a PyPI package, I think keeping the index arg is the right move. People using it wont always have control over when they insert which directories so it is likely to be useful to keep the capability of ending up with the directory order they want regardless. And, I still think that mirroring the ```sys.path.insert()``` signature is useful as well.