Refactor as package with subpackages

This makes it cleaner for introducing checks, fixes, tests, docs,
and tests in the future. Currently can be run like this:

  python -m csv_metadata_quality

CSV input and output paths are still hard coded.

See: https://dev.to/codemouse92/dead-simple-python-project-structure-and-imports-38c6

csv_metadata_quality/__main__.py

@@ -0,0 +1,4 @@
+from csv_metadata_quality import app
+
+if __name__ == '__main__':
+        app.run()