If PyTorch is build without caffe2 integration, do not package unusable
.py files/headers

Same is true about functorch - don't package it unless building with `functorch` (although, I wonder if we should remove this option at some point in the future)

Followup after https://github.com/pytorch/builder/pull/1181

Pull Request resolved: https://github.com/pytorch/pytorch/pull/87986
Approved by: https://github.com/seemethere

Co-authored-by: Nikita Shulga <nshulga@meta.com>