Do not fail when 'refine' targets an extension

Testing with NCS has revealed that aside from using 'augment' to target things inside an extension, they also use extensions in groupings and target their innards using 'refine'.

Implement the same workaround we have for augments for refine, too. That means determine that the target node exists to the extent we understand the model, issue a warning when we encounter an extension and ignore the refine statement.