Check top-level domain suggestion against domains

After correcting the top-level domain, the suggestion is sometimes
close enough to a full domain that accepting the suggestion results in
another bad email address. A previous commit tried to solve this
problem by changing the thresholds at which we suggest top-level
domains and full domains, but some edge cases remained. This
commit cuts out the middleman by directly checking for a full domain
suggestion before returning a top-level domain suggestion.