Feat: Directive to disable ancestor conftest.py deps

PR #3498 (Issue #3497) fixed things so that ancestor `conftest.py` files are automatically added as deps.

But... [Hyrum’s Law](https://www.hyrumslaw.com/). https://xkcd.com/1172/

Let's add a directive to disable this, defaulting to "ancestor `conftest.py` files are included in deps".