CMake: Fix exposing source files after target finalization

Calling _qt_internal_expose_source_file_to_ide after a target has
already been finalized did run the function's fallback code for targets
that are not finalized at all. That added the source files under a new
${target}_other_files target, which is a tad ugly in an IDE.

A situation where this occurs is for instance:
- qt_add_executable in a subdirectory
- adding additional files (licenses, documentation, .ts files) to the
  "main" target in the top-level CMakeLists.txt

The code flow is like this:
- enter subdirectory
  - qt6_add_executable(foo)
- leave subdirectory
  - qt6_finalize_target(foo)
    - calls _qt_internal_expose_deferred_files_to_ide(foo)
- _qt_internal_expose_source_file_to_ide(foo bar.txt)
  - sees that the target does not need finalization and runs the
    fallback code

Fix this by immediately running
_qt_internal_expose_deferred_files_to_ide if a target was already
finalized.

Task-number: QTBUG-127052
Change-Id: I1eda9acd48442ba35c6c61d9a185f6ed9ed6307f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit 469533fe8108cc56e90201919126502235c5cee6)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/Qt6CoreMacros.cmake

@@ -2115,8 +2115,17 @@ endfunction()
 function(_qt_internal_expose_source_file_to_ide target file)
     get_target_property(target_expects_finalization ${target} _qt_expects_finalization)
     if(target_expects_finalization AND CMAKE_VERSION VERSION_GREATER_EQUAL "3.19")
+        # The target is not yet finalized. The target finalizer will call the exposure function.
         set_property(TARGET ${target} APPEND PROPERTY _qt_deferred_files ${file})
         return()
+    else()
+        get_target_property(is_finalized "${target}" _qt_is_finalized)
+        if(is_finalized)
+            # The target already has been finalized. Run the exposure function immediately.
+            set_property(TARGET ${target} APPEND PROPERTY _qt_deferred_files ${file})
+            _qt_internal_expose_deferred_files_to_ide(${target})
+            return()
+        endif()
     endif()
 
     # Fallback for targets that are not finalized: Create fake target under which the file is added.