Flatten python requirements 2

[mcdonc]

I figured out how to use the output of a Python script during enterShell to flatten requirements and reinstall when they've changed.

It has a backwards imcompatibilty: "python.venv.requirements" now must be a string, and the string must be a devenv-root-relatvive path to a requirements file. This will break anyone who uses the option. Not sure how to signify this in the checkin without some sort of changelog file.

[domenkozar]

Ideally to land this we need import-from-derivation to work well. This is kind of blocked on #1548 unless we figure out a way for requirements to still be a types.path in another way.