enable "go to exact" for rustdoc

[Emilgardis]

I would really like something like go_to_exact in rustdoc that we could add for the :: redirects. I don't think we can just add go_to_first always because e.g. https://docs.rs/tokio?search=tokio::span&go_to_first=true goes to the tokio::spawn docs.

Originally posted by @Nemo157 in rust-lang/docs.rs#1830 (comment)

Add a query go_to_exact which will only use go_to_first if the match is "exact", what "exact" means should be determined.

[Nemo157]

There was also some related discussion on zulip: https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/go_to_first.3Dmaybe, the suggestions there are to have it as a tri-state on the existing flag like go_to_first=if-exact rather than a separate parameter.