Add warning when schema $ref is a tag

[eslavich]

It looks to me like $ref values were intended to be URIs or URI fragments, and it's only a quirk of the load_schema function in this library that tag values are resolved at all. Consider adding a check to the pytest plugin that fails when $ref is set to a tag, and a deprecation warning to load_schema so that we can encourage correct use of $ref.

[jdavies-st]

And this is because the tag_to_url_mapping and url_mapping` are always chained together and operate on both tags and URLs?