Fix fallback logic

From the commit message of 2db1fbd4fc:

As far as I can tell, the prior “add domain to sitelinksForPage only
once” logic was unnecessary, so we should be fine with dropping it and
making the set a list instead.

That was just me being an idiot – this logic is of course what makes
fallbacks work: if we already have an entry for a domain, don’t add
sitelinks from fallback item IDs as well. Restore this logic by
maintaining a separate map of domains that already have some sitelinks
in the result.