Fix up dependency checking (use 'dependencies' instead of 'external_dependencies' -> simpler/easier to remember

tests/data/example_module/__manifest__.py

@@ -2,7 +2,7 @@
     "name": "Example Module",
     "type": ["extractor"],
     "requires_setup": False,
-    "external_dependencies": {"python": ["loguru"]
+    "dependencies": {"python": ["loguru"]
                               },
     "configs": {
             "csv_file": {"default": "db.csv", "help": "CSV file name"}

tests/data/example_module/example_module.py

@@ -1,4 +1,4 @@
 from auto_archiver.core.extractor import Extractor
-
 class ExampleModule(Extractor):
-    pass
+    def download(self, item):
+        print("do something")