CMake: Add Qt6 forward compatible CMake API

Task-number: QTBUG-74137
Task-number: QTBUG-80477
Change-Id: Iaf88de57dd25f0a28c153667593203f75b6c472c
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

tools/qmlimportscanner/Qt5QmlImportScannerConfig.cmake.in

@@ -182,3 +182,13 @@ but this file does not exist.  Possible reasons include:
     endif()
 !!ENDIF // !isEmpty(CMAKE_STATIC_TYPE)
 endfunction()
+
+if(NOT QT_NO_CREATE_VERSIONLESS_FUNCTIONS)
+    function(qt_import_qml_plugins)
+        if(QT_DEFAULT_MAJOR_VERSION EQUAL 5)
+            qt5_import_qml_plugins(${ARGV})
+        elseif(QT_DEFAULT_MAJOR_VERSION EQUAL 6)
+            qt6_import_qml_plugins(${ARGV})
+        endif()
+    endfunction()
+endif()