I think the implementation is fine too (others will have to check it
more carefully) but I noticed that the promised functionality of -m
doesn't work yet: I created a file Lib/test/foo.py whose sole contents
was "from . import test_support". Then I tried to import this using
-m:

$ ./python.exe -m test.foo
Traceback (most recent call last):
 File "/Users/guido/p/Lib/runpy.py", line 104, in _run_module_as_main
   "__main__", fname, loader)
 File "/Users/guido/p/Lib/runpy.py", line 34, in _run_code
   exec code in run_globals
 File "/Users/guido/p/Lib/test/foo.py", line 1, in <module>
   from . import test_support
ValueError: Attempted relative import in non-package
$

What's missing here?