draft: fix!: use importlib instead of pkg_resources for modutil._is_namespace

This is a WIP and not retrocompatible.

The mess between pkg_resources and importlib and PEP 420 makes it very complicated to actually implement this thing with importlib.

Also this code only works if the package follows PEP 420, not the pkg_resources namespace approach.

I kinda want to burn down all this code honestly and we probably should just totally rewrite this module.