Allow installing python deps using pth files

[DolceTriade]

Currently, rules_pyvenv installs all the python deps by symlinking them in site-packages. However, in bazel, the python deps are actually added to the PYTHONPATH, which means that for some deps, just symlinking into site-packages doesn't work.

However, venv supports this via pth files. Would it be feasible to add a flag to py_venv to also create a .pth file to append these deps to the PYTHONPATH as well?

[alexeagle]

In https://github.com/aspect-build/rules_py we have this feature.