Backport fix for load_tests in package modules.

Discover was not executing load_tests at the package level, in
contrast to the docs which suggested it was possible. This is a
backport of the Python upstream proposed fix.

Change-Id: Ib952d5ce4ae1a3da0cd27dcf09315d20055d6b5a
Closes-Bug: http://bugs.python.org/issue16662