unzip the attached zip file and run main.py

expected output:

True
<module 'testext.sub' (namespace)>

actual output:
False
<module 'testext.sub' (namespace)>

So what?
If you follow these directions, https://docs.python.org/3.7/library/importlib.html#checking-if-a-module-can-be-imported , you will put a stale reference in sys.modules even though a fresh reference is already there. If you use that code on a package with a subpackage, the subpackage will not be set as an attribute on the package.