[caffe2] Factor libtorch_python_sources into exposed definition (#36005)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36005
Getting ovrsource pytorch working, requires single source list across ovrsource and fbsource to avoid build failures every time the source list changes.
This diff factors out libtorch_python_sources into a separate function (needs to be function because it uses glob which is disallowed at global scope)
Test Plan: CI
Reviewed By: malfet
Differential Revision: D20852072
fbshipit-source-id: 0e8ae3f6605e090e3ffdd6aa227fac905e7d9877
