Make libopencv python-independent; collapse CI jobs

[h-vetinari]

Alright, this is what I promised to look in to in #363. I could see that the default python standalone build will fail, and I patched that, but it'll probably take some iteration to get right.

The commits are intended to be reviewed in sequence, the overall diff is almost unreadable.

Closes #364

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[hmaarrfk]

curious to see what happens. Thanks!

[hmaarrfk]

maybe skip ffmpeg 5 builds?

[h-vetinari]

I don't understand why including some CMake modules would cause ninja not to be found, as it's clearly in the environment...

CMake Error:
  The detected version of Ninja () is less than the version of Ninja required
  by CMake (1.3).

[traversaro]

I don't understand why including some CMake modules would cause ninja not to be found, as it's clearly in the environment...

CMake Error:
  The detected version of Ninja () is less than the version of Ninja required
  by CMake (1.3).

I am no sure this is what is causing that error, but could this be related to the fact that if OPENCV_INITIAL_PASS is enabled, there is no call to cmake_minimum_required and project, that are required by any CMake project as first step?