7140: Store trait associated items in fst r=matklad a=SomeoneToIgnore
Store imported traits' associated function/methods and constants into `ImportMap.fst` and pefrorm the imports search on them.
This is a first step towards trait autoimport during completion functionality, the way I see it, after this PR, only a few major things are left to be done:
* store all traits' assoc items into fst, not only the ones in scope, as we do now. Any code pointers on how to do this are welcome 😄
* adjust a few modules in completions crate (`dot.rs`, `qualified_path.rs` at least) to query the import map, reusing the `import_assets` logic heavily
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With the current import and autoimport implementations, it looks like for a single query, we're either interested in either associated items lookup or in all other `fst` contents lookup, but never both simultaneously.
I would rather not split `fst` in two but add another `Query` parameter to separate those, but let me know if you have any ideas.
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
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