private boolean isPathField(Object builder, String methodName) {
        // Paths are stored as FileReference in Builder.
        return isFieldType(pathFieldSet, builder, methodName, FileReference.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