Restore symlinked Python source traversal

[Bben01]

Summary

Restore traversal of symlinked directories when maturin collects Python package sources for wheels and sdists.

I dont know if it was removed intentional or not. If it was, I could add some bool to the pyproject.toml that could toggle this behavior (and set the current default to false)

Why

In our team we package multiple Python packages that share some common code. We use symlinks for those shared directories and files so the source is maintained in one place instead of duplicated across package trees.

This worked previously (from what i remember - 1.7.0), but recent behavior no longer followed symlinked directories while building a pyo3 Rust + Python layout. Symlinked files were still included, but files inside symlinked directories were omitted from the wheel.

Fix

Use follow_links(true) when walking Python package sources for both wheel and sdist generation.

Validation

• Confirmed the new regression test fails before the fix: linked_dir/nested.py is missing from the wheel.
• Confirmed it passes after the fix with cargo test --test run -- pyo3_mixed_py_subdir_includes_symlinked_python_files.
• Ran cargo fmt --all and git diff --check.

AI disclosure

Fix was written by codex 5.5