Message 397749 - Python tracker

Additional problems I noticed while working on the refactor:

- There is a lot of overlap in behavior between the implementations of _run_script and _run_module (initializing private variables, setting mainpyfile, resetting the __main__ namespace).

- There are two block comments in _run_script whose behaviors apply to _run_module as well, but the block comments are not associated with those behaviors in _run_module.