This allows for a more graceful transition for projects using C++20
without scanner support (e.g., Clang 15 or GCC 13). While newer
compilers will (needlessly) scan, it allows C++20-using projects to use
older compilers without having to set `CMAKE_CXX_SCAN_FOR_MODULES` to
support newer CMake minimum versions.
Fixes: #25357
Now that scanning support is no longer experimental, the logic for
whether or not to scan C++ 20 sources is now important because all
projects are now exposted to the logic. Make the scanning rules explicit
in the documentation and rework the queries to localize all of the
associated logic.
A policy to handle the ultimate fallback logic will be implemented in a
following commit.
