bpo-44461: Check early that a pdb target is valid for execution. (#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 <iritkatriel@yahoo.com>
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>