Related to #23639 but not quite the same.

If a python package launches python, the child's PYTHONPATH will know nothing of the dependencies of the parent. This is almost never what you want. I think this is because rather than setting PYTHONPATH, we use site inside python to set the path for the current interpreter. Spawning new child interpreters won't get any of that stuff.

My current inclination would be to add a very simple line to the wrapper shim we have today:

os.environ['PYTHONPATH'] = ":".join(sys.path)

Right after we call reduce to add all the paths. That would ensure that all children inherit the right environment. Any thoughts?

cc @FRidh