CMake: Fix finding standalone parts config file for yocto

When building standalone tests of qtsvg targeting yocto, the
standalone parts config file was not found.

That's because it specifies a new CMAKE_STAGING_PREFIX for each built
repo, and the QtSvgTestsConfig.cmake file will be located there,
rather than in QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX, where
the QtBuildInternalsConfig.cmake file is.

So in this case, we always have to prefer looking into
CMAKE_STAGING_PREFIX for the file.

Amends 62905163bf

Task-number: QTBUG-90820
Task-number: QTBUG-96232
Change-Id: I8902381afa4267e40dfb2ad47e44285a806a35e2
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>

cmake/QtBuildRepoHelpers.cmake

@@ -497,7 +497,15 @@ function(qt_get_standalone_parts_config_files_path out_var)
     set(dir_name "StandaloneTests")
 
     set(path_suffix "${INSTALL_LIBDIR}/cmake/${INSTALL_CMAKE_NAMESPACE}BuildInternals/${dir_name}")
-    set(path "${QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX}/${path_suffix}")
+
+    # Each repo's standalone parts might be configured with a unique CMAKE_STAGING_PREFIX,
+    # different from any previous one, and it might not coincide with where the BuildInternals
+    # config file is.
+    if(QT_WILL_INSTALL AND CMAKE_STAGING_PREFIX)
+        qt_path_join(path "${CMAKE_STAGING_PREFIX}" "${path_suffix}")
+    else()
+        qt_path_join(path "${QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX}" "${path_suffix}")
+    endif()
 
     set("${out_var}" "${path}" PARENT_SCOPE)
 endfunction()