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/__manifest__.py

@@ -0,0 +1,10 @@
+{
+    "name": "Example Module",
+    "type": ["extractor"],
+    "requires_setup": False,
+    "external_dependencies": {"python": ["loguru"]
+                              },
+    "configs": {
+            "csv_file": {"default": "db.csv", "help": "CSV file name"}
+        },
+}