If PYTHON_INCLUDE_PATH is put into the cache, then it will always
override whatever might be found and PYTHON_INCLUDE_DIR is never given a
chance to find something different. It being marked as INTERNAL also
means that it cannot be changed without editing CMakeCache.txt directly.

Basically, the scenario is that if the Python version is changed, then
deleting PYTHON_INCLUDE_DIR doesn't work because any cached
PYTHON_INCLUDE_PATH variable is set before find_path is even called. Any
build tree using a previous version will still need either manual
removal of PYTHON_INCLUDE_PATH or a complete reconfigure, but in the
future changing the Python version can be accomplished by deleting
PYTHON_INCLUDE_DIR and reconfiguring with the new version.