If there was a mod and non-mod meta tag for a given prefix,
the meta tag extractor was already dropping the non-mod meta tag.
But we might have mod and non-mod meta tags with different
prefixes, in which case the mod tag should prevail when both match.

Fixes #26200.

Change-Id: I17ab361338e270b9fa03999ad1954f2bbe0f5017
Reviewed-on: https://go-review.googlesource.com/124714Reviewed-by: Bryan C. Mills <bcmills@google.com>