The ejb-dist-cache layer includes the 'private-interface' feature group. Its declaration of that wraps the element by stating its origin is the wildfly-servlet feature pack, but that wrapping is not necessary.

Refs to wildfly-servlet prevent the layer-spec source being used by the EE 9 wildfly-preview FP which doesn't depend on wildfly-servlet. Other such refs were removed as part of earlier EE 9 work but this one crept back in via a topic branch for the WF 21 galleon layers work.