private void validateNeighbors(Shape shape, List<Relationship> relationships, List<ValidationEvent> events) {
        String namespace = shape.getId().getNamespace();
        for (Relationship rel : relationships) {
            if (!rel.getShape().getId().getNamespace().equals(namespace)) {
                events.add(getPrivateAccessValidationEvent(rel));
            }
        }
    }
Ensures that shapes in separate namespaces don't refer to shapes in other
namespaces that are marked as private.
smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/PrivateAccessValidator.java