This method is used to filter away extension attributes that do not extend 'normally' declared attributes for current node.
It is invoked for each attribute collected initially in AbstractComponent.getAttributes(...) methods. Current implementation of checkExtended() requests for all attribute definitions in all libraries at each call. That makes N*N dependency of the number of queries on the number N of attribute definitions.
We can cache the set of names of non-extension attributes obtained at the first invocation.