CMake: Fix Gui package to auto look up IntegrityPlatformGraphics

When building non-qtbase tqtc repos in CI, we don't load a lot of
Plugin Config files in static builds due to project names having a
tqtc- prefix. This is similar to the issue and workaround that was
done in 4c6292686259e4e232f29cb6fd6c79065e9fa96d for qtserialport.

The specific issue here is the following error:

CMake Error at Qt6Gui/Qt6GuiTargets.cmake:61 (set_target_properties):
  The link interface of target "Qt6::Gui" contains:

    IntegrityPlatformGraphics::IntegrityPlatformGraphics

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  /home/qt/work/install/target/lib/cmake/Qt6Gui/Qt6GuiConfig.cmake:52
   (include)
  /home/qt/work/install/target/lib/cmake/Qt6/Qt6Config.cmake:157
  (find_package)
  CMakeLists.txt:15 (find_package)

To work around the issue, explicitly record a dependency on the
IntegrityPlatformGraphics target for Gui when building on INTEGRITY.

The underlying issue is sadly still not fixed.

Change-Id: I9a9cff05d036f224aab8083ad6bc8b8e568abd8b
Pick-to: 6.6
Task-number: QTBUG-102883
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>

src/gui/CMakeLists.txt

@@ -338,7 +338,8 @@ if(QT_FEATURE_opengl)
         target_link_libraries(Gui PUBLIC GLESv2::GLESv2)
 
         if(INTEGRITY AND _qt_igy_gui_libs)
-            find_package(IntegrityPlatformGraphics)
+            qt_find_package(IntegrityPlatformGraphics
+                PROVIDED_TARGETS IntegrityPlatformGraphics::IntegrityPlatformGraphics)
             target_link_libraries(Gui
                 INTERFACE $<LINK_ONLY:IntegrityPlatformGraphics::IntegrityPlatformGraphics>)
         endif()