Tidy ups + unit tests:

1. Allow loading modules from --module_paths=/extra/path/here
2. Improved unit tests for module loading
3. Further small tidy ups/clean ups

tests/data/example_module/example_module.py

@@ -0,0 +1,4 @@
+from auto_archiver.core.extractor import Extractor
+
+class ExampleModule(Extractor):
+    pass