Thanks.  It looks like the issue with the latest patch is caused by side effects of calling importlib.import_module().

Working from the patch, I got it to the point where inserting the following four lines somewhere in the code--

    try:
        importlib.import_module('foo__doesnotexist')
    except:
        pass

caused the exception raised by the following line--

    module = importlib.import_module('package_foo2.subpackage.no_exist')

to change from this--

  ...
  File "<frozen importlib._bootstrap>", line 1250, in _find_and_load_unlocked
ImportError: No module named 'package_foo2.subpackage.no_exist'

to this--

  ...
  File "..../Lib/importlib/_bootstrap.py", line 1257, in _find_and_load_unlocked
    raise ImportError(_ERR_MSG.format(name), name=name)
ImportError: No module named 'package_foo2'

It looks like this issue is cropping up in the tests because the test code dynamically adds packages to directories that importlib may already have examined.

In the reduced test case I was creating to examine the issue, I found that inserting a call to importlib.invalidate_caches() at an appropriate location resolved the issue.

Should loadTestsFromName() call importlib.invalidate_caches() in the new patch implementation, or should the test code be aware of that aspect of loadTestsFromName()'s behavior and be adjusted accordingly (e.g. by creating the dynamically-added packages in more isolated directories)?  For backwards compatibility reasons, how does loadTestsFromName() currently behave in this regard (i.e. does importlib.import_module() behave the same as __import__ with respect to caching)?