4819: Add an FST index to `ImportMap` and use it to speed up auto import r=matklad a=jonas-schievink
For the importing crate, we still use the symbol index, but I've modified it to only look at files that comprise that crate (instead of the whole workspace).
Oh, and since now the symbol query limit is respected correctly, it's possible that some results from the local crate now disappear if there are many matches.
Fixes https://github.com/rust-analyzer/rust-analyzer/issues/4763
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>
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