Failing top-level imports hide modules with the same
name in packages.

The attached zipfile contains a script main.py and a
simple contrived package demonstrating the problem.

If the zip-archive is unpacked to some directory, and
modulefinder is run in this directory, it prints this:

c:\sf\test>py24 -m modulefinder main.py

  Name                      File
  ----                      ----
m __main__                  main.py
P a                         a\__init__.py
P a.c                       a\c\__init__.py

Missing modules:
? spam imported from __main__, a

c:\sf\test>

With the attached patch, the output is like this:

c:\sf\test>py24 -m modulefinder main.py

  Name                      File
  ----                      ----
m __main__                  main.py
P a                         a\__init__.py
P a.c                       a\c\__init__.py
m a.c.spam                  a\c\spam.py
m a.spam                    a\spam.py

Missing modules:
? spam imported from __main__

c:\sf\test>