setup: always create a support dir and populate it with a site-packages and add same...

setup: always create a support dir and populate it with a site-packages and add same to the PYTHONPATH, just in case someone is going to do "build", "develop", or "test" or something else which triggers a build
I think there must be a much better solution for this -- probably to fix setuptools #54 and ship our own fork of setuptools and rely on it.