Some filters don't respect URI synonyms

[btrask]

When the user performs a query pertaining to a URI, for example "show me everything that links here," they are talking about file or a set of files sharing that URI, not that URI itself. A URI can match multiple files, and each file can have multiple URIs. Thus we need to do extra work to find synonyms.

Currently this is only done for the links-to filter, and there is some performance overhead because we literally do a union of each synonym. It's extra complexity to apply it to different filters, and depending on the filter the overhead might be significant.

Synonym matching probably shouldn't be done for target queries, since meta-files target URIs, not files.