tags: only return 'alltags' in 'findglobaltags'
This is minor update along the way. We simplify the 'findglobaltags' function to
only return the tags. Since no existing data is reused, we know that all tags
returned are global and we can let the caller get that information if it cares
about it.
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273ce12ad8f1` will report some style violations on a very old commit).