Try to fix namespace packages are not handled correctly issue

It appears that PyInstaller can't handle namespace packages correctly. For example

* foo.pkg1
* foo.pkg2

When the foo is a namespace package, module or packages in pkg1 or pkg2 can't be find correctly. 

To solve that problem, I add few lines in depend/modules.py. It uses 

    pkgutil.extend_path

function for extending other package paths.  Not sure is there side effect by doing this, but at least, this solution works for me.