private boolean isModelField(Object builder, String methodName) {
        // Models are stored as ModelReference in Builder.
        return isFieldType(modelFieldSet, builder, methodName, ModelReference.class);
    }
Checks if this field is of type 'path', in which
case some special handling might be needed. Caches the result.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java