[3.13] gh-59022: Added tests for `pkgutil.extend_path` (GH-121673) (GH-121951)

This adds tests for the documented behaviour of `pkgutil.extend_path`
regarding different argument types as well as for `*.pkg` files.
(cherry picked from commit 8f2532168b4630e413a42ccb5e458708808702ea)

Co-authored-by: Andreas Stocker <andreas@stocker.co.it>