Windows: enforce activation of our PYTHONPATH's eggs

Since setuptools 49.0.0 [1], their easy_install command no longer installs
site.py to PYTHONPATH directories. This didn't hit me (or anyone using,
as per 3rdparty, stock Python 3.8.1) because that version ships
Setuptools 41; but anyone using v49 onwards would've run on importlib
errors when trying to use sip-build or any of our 3rdparty tools.
This bug can be easily triggered by making a virtual environment using
virtualenv and then executing build.cmd inside it.

This commit reuses Iván's sitecustomize.py for macOS and adds
the entry suggested in [2] to inject our directory.

While at it, let's ensure all PYTHONPATH entries are sanitized and
stored in KRITA_PYTHONPATH, not just ours.

[1]: https://setuptools.readthedocs.io/en/latest/history.html#v49-0-0

[2]: https://github.com/pypa/setuptools/issues/2165

CCMAIL: kimageshop@kde.org