* Splitting out pybind11/stl/filesystem.h.
To solve breakages like: https://github.com/deepmind/open_spiel/runs/2999582108
Mostly following the suggestion here: https://github.com/pybind/pybind11/pull/2730#issuecomment-750507575
Except using pybind11/stl/filesystem.h instead of pybind11/stlfs.h, as decided via chat.
stl.h restored to the exact state before merging PR #2730 via:
```
git checkout 733f8de24feed964f96b639a0a44247f46bed868 stl.h
```
* Properly including new stl subdirectory in pip wheel config.
This now passes interactively:
```
pytest tests/extra_python_package/
```
* iwyu cleanup.
iwyuh.py -c -std=c++17 -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/include/python3.9 -I/usr/include/eigen3 include/pybind11/stl/filesystem.h
* Adding PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL.
* Eliminating else after return.