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/core/misc/python/package/utils/__init__.py

@@ -0,0 +1,14 @@
+from collections import namedtuple
+
+import cv2
+
+
+NativeMethodPatchedResult = namedtuple("NativeMethodPatchedResult",
+                                       ("py", "native"))
+
+
+def testOverwriteNativeMethod(arg):
+    return NativeMethodPatchedResult(
+        arg + 1,
+        cv2.utils._native.testOverwriteNativeMethod(arg)
+    )