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.
It turns out that `addsitedir` is already resolving relative path, so I'm not sure the `join` dance is needed.

```
>>> import site
>>> site.addsitedir('env')
>>> import sys
>>> sys.path[-1]
'/tmp/app/env'
```