I'm wondering if there might be a simpler option: use rpartition() to strip off any trailing segment (whether that's "__main__" or not), and then always do a plain dynamic import of that package (if any). Something like the following at the start of _get_module_details():

    pkg_name, is_submodule, submodule = mod_name.rpartition(".")
    if is_submodule:
        __import__(pkg_name)

The key is that we *don't* want to be relying on the fact find_spec() will import parent packages implicitly.