refactor!: rename video_producers → producers

Since it no longer simply produces videos this name is preferred

Also update tests to use updated module name

test/test_producers.py

@@ -1,6 +1,6 @@
 """test_producers.py -- test the producers in the pipeline (eg ffmpeg, visualisation, json)"""
 import unittest
-import pipeline.video_producers as producers
+import pipeline.producers as producers
 import test.mocks as mocks