Fix standalone headersclean_check targets

If module depends on some module Private part only the sync_headers
dependencies from this public part of this Private module is skipped.
Check if the module is Private and use the public module to read the
_qt_internal_sync_headers_target property.

Amends 0006a564c8

Fixes: QTBUG-115712
Pick-to: 6.6
Change-Id: I4678e2a6c9ec8f80862a97d48f0a3b4214ae2307
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtTargetHelpers.cmake

@@ -119,6 +119,10 @@ function(qt_internal_extend_target target)
             # heuristic way of building the dependency tree between the _sync_headers targets of
             # different Qt modules.
             if(TARGET "${lib}")
+                get_target_property(is_private ${lib} _qt_is_private_module)
+                if(is_private)
+                    get_target_property(lib ${lib} _qt_public_module_target_name)
+                endif()
                 set(out_genex "$<TARGET_PROPERTY:${lib},_qt_internal_sync_headers_target>")
                 set_property(TARGET ${target}
                     APPEND PROPERTY _qt_internal_sync_headers_deps "${out_genex}")