calib3d is split into 3d, calib and stereo

[vpisarev]

relates to #18267

At once, some of the old code has been finally ported from OpenCV 1.x C API to C++ API.
Note that this pull request is just beginning of 3D functionality refactoring. Probably, it's too radical in some places, e.g. cv::findHomography() is now spelled as cv3d::findHomography(), which may break a lot of users' code; probably, some of the most popular functionality from 3D, calib and stereo module should be exposed in cv namespace, not the nested namespaces.

There is satellite pull request in opencv_contrib:

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• [N/A] There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[vpisarev]

closed in favor of #18940