Merge pull request #20611 from VadimLevin:dev/vlevin/pure-python-modules

* feat: OpenCV extension with pure Python modules

* feat: cv2 is now a Python package instead of extension module

Python package cv2 now can handle both Python and C extension modules
properly without additional "subfolders" like "_extra_py_code".

* feat: can call native function from its reimplementation in Python

modules/python/package/extra_modules/misc/__init__.py

@@ -0,0 +1 @@
+from .version import get_ocv_version

modules/python/package/extra_modules/misc/version.py

@@ -0,0 +1,5 @@
+import cv2
+
+
+def get_ocv_version():
+    return getattr(cv2, "__version__", "unavailable")