PY-19738: Django test targets should be resolved against source roots too, not only...

PY-19738: Django test targets should be resolved against source roots too, not only against django project root

Django target is something like "spam.eggs.MyClass". That means "spam" is package inside Django project root or in source root. Django only adds its project root to sys.path automatically, but user may add some other folders dynamically (see base.py, local.py in this commit) and mark folder as "source". We should support this config.