After r12950, load_app does not return models modules from eggs
#13334 points out a problem loading template tags from eggs. Since the change (to use
imp.find_module) that introduced that problem was the inspiration for r12950, the r12950 code has the same problem, only for r12950 the problem involves loading models modules from apps in eggs. Ramiro pointed to a patch on #9427 that attempts to find modules loaded by eggs, but I could not quite get that code to work. I'll attach a patch that I did get to work, with tests. This patch introduces a utility function to answer the question: does this loaded module have a submodule named x? This seems to be something we could use in a few places.