CMake: Fix Gui package to auto look up IntegrityPlatformGraphics v2

The previous attempt to record a dependency on the
IntegrityPlatformGraphics target for the Gui target was insufficient.

Aside from a qt_find_package(PROVIDED_TARGETS) call, we also need to
use qt_internal_extend_target(Gui PRIVATE IntegrityPlatformGraphics)
to ensure the dependency is written into the Qt6GuiDependencies.cmake
file.

Replace the target_link_libraries call with qt_internal_extend_target,
and remove the qt_find_package all together. A qt_find_package call
in src/gui/configure.cmake already exists, so the one in
CMakeLists.txt is redundant and can be removed.

Finally the
  qt_internal_extend_target(Gui PRIVATE IntegrityPlatformGraphics)
call will also result in a
  target_link_libraries(Gui INTERFACE
  $<LINK_ONLY:IntegrityPlatformGraphics::IntegrityPlatformGraphics>)
because Gui is a static library when building on Integrity, so the
transitive requirement will still be passed along to user projects.

Amends c03eb94c8a
Amends 8116fdde1c

Pick-to: 6.6
Fixes: QTBUG-118051
Task-number: QTBUG-102883
Change-Id: Ic962df94a20071d3f2459e705dbafaca0319a638
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/gui/CMakeLists.txt

@@ -338,10 +338,10 @@ if(QT_FEATURE_opengl)
         target_link_libraries(Gui PUBLIC GLESv2::GLESv2)
 
         if(INTEGRITY AND _qt_igy_gui_libs)
-            qt_find_package(IntegrityPlatformGraphics
-                PROVIDED_TARGETS IntegrityPlatformGraphics::IntegrityPlatformGraphics)
-            target_link_libraries(Gui
-                INTERFACE $<LINK_ONLY:IntegrityPlatformGraphics::IntegrityPlatformGraphics>)
+            qt_internal_extend_target(Gui
+                LIBRARIES
+                    IntegrityPlatformGraphics::IntegrityPlatformGraphics
+            )
         endif()
 
     elseif(NOT QT_FEATURE_opengl_dynamic)