4729: Hover actions r=matklad a=vsrs
This PR adds a `hoverActions` LSP extension and a `Go to Implementations` action as an example:

4748: Add an `ImportMap` and use it to resolve item paths in `find_path` r=matklad a=jonas-schievink
Removes the "go faster" queries I added in https://github.com/rust-analyzer/rust-analyzer/pull/4501 and https://github.com/rust-analyzer/rust-analyzer/pull/4506. I've checked this PR on the rustc code base and the assists are still fast.
This should fix https://github.com/rust-analyzer/rust-analyzer/issues/4515.
Note that this does introduce a change in behavior: We now always refer to items defined in external crates using paths through the external crate. Previously we could also use a local path (if for example the extern crate was reexported locally), as seen in the changed test. If that is undesired I can fix that, but the test didn't say why the previous behavior would be preferable.
Co-authored-by: vsrs <vit@conrlab.com>
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>