Split finding <dfn> elements from using them to link <a> tags.

This lets other modules use the definitionMap.

I'm using this in https://github.com/jyasskin/respec/commit/0f7e2e135ac91cacb72a8e4351afe00582c29bcf#diff-a7fc04791d8afbe7bc43cf038be115fbR589 to avoid needing to re-compute the set of `<dfn>` titles, but I could re-do the scan there if this refactoring is a bad idea.