Fix duplicate invalidTag issue generation

[happy5214]

As a result of fixes made in #45, node names which do not appear in the schema in any form now generate two invalidTag issues with basically the same content, one from short-to-long conversion and the other from tag validation. Since we can't eliminate either check entirely (since they don't completely overlap), we'll need to decide whether to conflate the issues (i.e. merge them) or keep both (perhaps distinguishing them with a field introduced for #46).