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.
we could also have a variant that add a virtualenv site-packages as a sitedir, see:
https://gist.github.com/proppy/a1b251c43bd39c9e8b90#file-appengine_config-py-L22