Symlink runfiles tree so that you can do a dev/test iteration without rebuilding PEX

[c4urself]

With py_test (PAR-based) rules I can dev/test iterate on an individual test by running the following:

1. bazel test //foo:foo_tests
2. find and update an issue in dependency bar.py
3. run bazel test //foo:foo_tests without rebuilding the PAR because the runfiles are symlinked

The iteration cycle (1 and 2 above) is fast because it essentially lays out the zipfile in the runfiles directory and symlinks files that I may want to edit.

With the PEX rules I cannot do this as the only files that are symlinked are the srcs files from the test itself. This means that I have to rebuild the PEX each time I have to edit a dependent file which makes life quite tedious for any serious development.

What should happen is that the runfiles tree is created similarly to what PAR does.

[benley]

I agree that would be nice - the challenge lies in keeping all the wheels/eggs/setuptools-stuff dependencies available. We could make a script wrapper that sets PEX_PATH to have pex get dependencies from the previously-built .pex, but I'm not sure how to make it keep a path outside of the .pex in sys.path after pex initializes itself. Any suggestions?