Introduce resources_functions.prf with the test function
qtFlattenResources which ensures that RESOURCES has a flat structure,
e.g. only contains *.qrc entries.
This can be called by other .prf files like qtquickcompiler.prf
without disturbing the ability to extend RESOURCES in other .prf files.
Task-number: QTBUG-79672
Change-Id: I43246e40f5ea52a9d9c917a1bd781aeeb1304acc
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
