Build the Python 3 cv2 module in lib/python3/, to avoid potential naming
conflicts with the Python 2 bindings.
The Python 2 bindings are placed directly in lib/, where they are
required for the Buildbot to successfully execute the Python tests.
If both Python 2 and Python 3 are found, then build bindings for both of
them during the build process. Currently, one version of Python is
detected automatically, and building for the other requires changes the
CMake config.
The largest chunk of this change generalizes OpenCVDetectPython.cmake to
find both a Python 2 and Python 3 version of Python. Secondly, the
opencv_python module is split into two modules, opencv_python2 and
opencv_python3. Both are built from the same source. but for different
versions of Python.
