Create base package that does not depend on OpenCV variant

[Breakthrough]

OpenCV ships 4 variants:

• opencv-python
• opencv-contrib-python
• opencv-python-headless
• opencv-contrib-python-headless

None of these can be combined, and right now to simplify things I made scenedetect depend on opencv-python and the -headless variant depend on opencv-python-headless.

We might need to further split out the packages into scenedetect-core so that downstream projects can decide which exact variant they want to include. That way scenedetect itself can continue to depend on opencv-python while other packages (e.g. dvr-scan) can include the contrib variant gracefully.