fix: Stop using PYTHONPATH to set up sys.path

[philsc]

This patch is an experiment for setting up sys.path inside
the final Python process instead of letting the runtime do it
via PYTHONPATH.

The motivation here is to address these 2 issues:

• sys.path[0] breaks out of runfile tree. #382
• using all_requirements causes OSError: [Errno 7] Argument list too long #792

Try it with:

$ bazel run //tools:wheelmaker -- --help

Look at the generated files.

[aignas]

Do you think it would help #1174 as well?

[philsc]

Do you think it would help #1174 as well?

Not as-is, no. But I think it could. In the python/private/common/entrypoint.tmpl.py file we could add an import hook that tries to resolve imports with the repository name.