Add the following variables:
* Python_EXECUTABLE_DEBUG
* Python_INTERPRETER
* Python_DEBUG_POSTFIX
and target Python::InterpreterDebug.
python_add_library() command Manage DEBUG_POSTFIX target property based
on the value of Python_DEBUG_POSTFIX variable.
Fixes: #25874
Do not set the policy version before recording our internal macros such
as `__Python_add_library`. Otherwise callers get our policy version
instead of theirs. Instead just set the specific policies we need.
Also fix one case in our test suite where we were accidentally
relying on the policy version to be set by `FindPython`.
Fixes: #21042
