@Override
        public String getMessage() {
            return deque.stream()
                    .map(SDDocumentType::getName)
                    .collect(joining("->", "Document dependency cycle detected: ", "."));
        }
Validates that there are no cycles between document types (exception: self-reference is allowed).
Example: if document B inherits A, then A cannot have a document reference to B.

@author bjorncs
config-model/src/main/java/com/yahoo/schema/DocumentGraphValidator.java