Detect broken automatic internal links when definition changes

[dontcallmedom]

When a spec changes e.g. the for-scope of its <dfn> (e.g. change <dfn for=bar>foo</dfn> to <dfn for=baz>foo</dfn>), this may break the autolinks used inside that document; when bikeshed knows of the former target (bar/foo) in a published version of the document via webref, it won't complain if {{bar/foo}} remains used in the defining document.

Ideally, bikeshed would detect that the document being generated is a version of the one where it knows the definition from, and would report {{foo/bar}} as broken.

see whatwg/xhr#389 (review) for a recent example