One further note and then I'll shut up and wait to hear your thoughts: if the file-module case I mentioned above (`foo.py`) is something we need to support, it would be easy to support with a fallback to `os.path.dirname(app_module.__file__)` if the module has no `__path__`.

So in summary, it seems possible to me that we could support everything we need to with the combo of supporting len-1 `__path__` and `__file__`, and allow anything that doesn't have either of those to be an immediate error.

But it's quite likely I'm wrong! You've spent a lot more time looking at this in the last month than I have.