Sketch: Use nested venvs to manage python dependencies

[olantwin]

• Allow separating base and analysis/user packages
• Make requirements more modular
• Maintain isolation at build and run time

References:
https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/
https://stackoverflow.com/questions/75514895/creating-a-cheap-overlay-python-virtual-environment-on-top-of-current-one-with
https://stackoverflow.com/questions/74436125/cascading-virtual-python-environnements
https://stackoverflow.com/questions/61019081/is-it-possible-to-create-nested-virtual-environments-for-python
https://docs.python.org/3/library/site.html

I'll look into this eventually, help would be very welcome to speed this along.