Warn if user passes lib/cmake/Qt6 in CMAKE_PREFIX_PATH

Users on the forum.qt.io post issues like not being able to configure Qt applications using Qt6 because they end up passing  C:\Qt\6.3.1\msvc2019_64\lib\cmake\Qt6 which doesn't works as they would expect.

One example building qt from sources - CMake Error at cmake/FindQt5.cmake

It would be nice if CMake would take these paths into consideration, but that would probably never happen.

But, we can have a look at the CMAKE_PREFIX_PATH and if it contains lib/cmake/Qt6 then issue a warning with the message to try without it.