Allow skipping/ignoring installed Qt modules from setup.py

I mentioned this first on PYSIDE-502 which I didn't realise was fixed already (sorry about that!).

In summary I have an application with an embedded python interpreter and I want to use PySide2 to expose only QtCore, QtGui and QtWidgets to the scripts to allow them to extend the UI.

However currently PySide2 always compiles in support for every module that it finds, and this means on windows I end up having to distribute more Qt modules that I don't use or want like QtQml and QtNetwork, because support has been compiled so there's a dependency on them.

Everything works fine if I pretend the other modules aren't found by hacking the CMakeLists.txt, but it would be nice to have the ability to exclude modules I don't want from the build process.