gh-75723: Fix double evaluation of .pth and .site files in venvs (#149583)

* Idempotent `.pth` execution in `site.addsitedir`
* potentially fix monkeypatch leak?

* fix blind copy paste of recommendation

* Update 2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst

* fix implicit merge conflict with 24c4aecc1674414d3dc3238625802778c4ad29d2

* Add failing tests for gh-75723

Based on @asottle branch !asottle-gh-75723 but refactored for `main`.
This will need a different backport.

* Repair gh-75723

The original fix is here: https://github.com/python/cpython/pull/147951
but I'm refactoring a bit for `main`.

* Refactor _make_mod() so we can use it to create package modules too

* Add myself to CODEOWNERS for the site module

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Co-authored-by: anthony sottile <asottile@umich.edu>
Co-authored-by: Filipe Laíns <lains@riseup.net>