[3.9] bpo-44461: Check early that a pdb target is valid for execution. (GH-27227...

[3.9] bpo-44461: Check early that a pdb target is valid for execution. (GH-27227) (GH-27400)

* [3.9] bpo-44461: Check early that a pdb target is valid for execution. (GH-27227)

* bpo-44461: Fix bug with pdb's handling of import error due to a package which does not have a __main__ module

* 📜🤖 Added by blurb_it.

* remove "else"

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>
* If running as a module, first check that it can run as a module. Alternate fix for bpo-44461.

Co-authored-by: Irit Katriel
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>.
(cherry picked from commit ee03bad25e83b00ba5fc2a0265b48c6286e6b3f7)

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>
* Ensure os_helper is imported.

* Actually, os_helper doesn't exist yet. Just reference rmtree from support.