CI: Check private function not used across modules

[ShaharNaveh]

• xref (WIP) CI: Check that private functions are not used across modules #32942 (comment)
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @MomIsBestFriend! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file scripts/validate_unwanted_patterns.py:

Line 154:9: E266 too many leading '#' for block comment

[jbrockmendel]

@MomIsBestFriend can i convince you to pick this and/or #33394 back up? I found myself manually tracking down incorrectly-private functions today, would be nice to avoid that need.