modulefinder wasn't searching for dependencies in the script dir.

It's not capable of detecting the sys.path manipulations scripts do
internally neither.

This change fixes the first issue, and hacks around the second.

Honestly, I've come to the conclusion that automatic Python dependency it will always be
too brittle.   I think we should start manually typing the dependencies
like we do in automake.  At very least it will enable any person to
eyeball and spot/fix missing dependencies, without dig into SCons internals.