Cache flattened parents during model building

During model building, we do not cache any hierarchy inheritance due to profiles that may be activated based on properties from the child projects.
So there's no way to cache the effective parent and just inject it into the child, the whole hierarchy need to be taken into account.

However, flattening the parent (i.e. inject the flattened parent's parent into the raw parent) should provide a parent model which has no parents anymore, not activated and not interpolated, but which can be cached and reused.