CMake: Allow setting Qt6_DIR to find packages when not cross-compiling

So far we always recommended that developers set CMAKE_PREFIX_PATH to
point to the location of the Qt installation so that Qt packages are
found by CMake.

In Qt5 one could also set the Qt5_DIR variable to $qt/lib/cmake/Qt5
to allow the following signature to work:
 find_package(Qt5 COMPONENTS Core)

This was not sufficient in Qt6 because the CoreTools dependency would
not be found.

Fix this by also looking into the parent folder of
CMAKE_CURRENT_LIST_DIR as well as _qt_cmake_dir, like we already do
for regular non-Tools packages in _qt_internal_find_dependencies.

Note that setting Qt6_DIR is not sufficient for cross-compilation if
the qt.toolchain.cmake file is not used.
Aside from platform specific options that would have to be passed
manually to CMake (like -DCMAKE_SYSTEM_NAME=iOS for iOS) and
passing a QT_HOST_PATH value, the code would also need to be
adapted to do the necessary CMAKE_FIND_ROOT_PATH manipulations
to ensure that dependent packages are found.

Pick-to: 6.4
Task-number: QTBUG-97615
Change-Id: I10c419632d43bb929e184bab3b9d3d27a35ea87a
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

cmake/QtPublicDependencyHelpers.cmake

@@ -58,6 +58,8 @@ macro(_qt_internal_find_tool_dependencies target target_dep_list)
             ${__qt_${target}_version}
             ${__qt_${target}_find_package_args}
             PATHS
+                "${CMAKE_CURRENT_LIST_DIR}/.."
+                "${_qt_cmake_dir}"
                 ${_qt_additional_packages_prefix_paths}
         )
         if (NOT ${__qt_${target}_pkg}_FOUND)