CMake: Pierce through LINK_ONLY deps in finalizer dep traversal

Ensure that the finalizer approach of
__qt_internal_propagate_object_library considers $<LINK_ONLY:>
libraries when traversing the dependencies of a target.

The issue was discovered when using the Quick.Shapes QML module in a
static build. The module has both a backing library and a plugin.
The backing library has some resource objects associated with it.
When the targets are exported, the plugin INTERFACE_LINK_LIBRARIES
has a $<LINK_ONLY:QuickShapes> dependency.

This ensures that the library will be linked, but depending on which
linking approach in __qt_internal_propagate_object_library is used,
the resources might not be linked to the final executable.

The resources are linked correctly when using the
target_link_libraries approach, but not when using the finalizer or
target_sources approach.

This change fixes the finalizer approach, but the target_sources
approach is still broken.

Amends a1fd4f51ad

Pick-to: 6.2
Change-Id: Ifbb91a17d388c3dc4263e17ec0d3bd5627b57cb4
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtPublicTargetHelpers.cmake

@@ -177,6 +177,11 @@ function(__qt_internal_collect_object_libraries_recursively out_var target initi
 
     set(object_libraries "")
     foreach(lib IN LISTS libs interface_libs)
+        # Extract possible target from exported LINK_ONLY dependencies.
+        # This is super important for traversing backing library dependencies of qml plugins.
+        if(lib MATCHES "^\\$<LINK_ONLY:(.*)>$")
+            set(lib "${CMAKE_MATCH_1}")
+        endif()
         if(TARGET ${lib})
             get_target_property(aliased_target ${lib} ALIASED_TARGET)
             if(aliased_target)