Merge #4729 #4748

4729: Hover actions r=matklad a=vsrs

This PR adds a `hoverActions` LSP extension and a `Go to Implementations` action as an example:
![hover_actions_impl](https://user-images.githubusercontent.com/62505555/83335732-6d9de280-a2b7-11ea-8cc3-75253d062fe0.gif)



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>

crates/ra_db/src/lib.rs

@@ -11,8 +11,8 @@ use ra_syntax::{ast, Parse, SourceFile, TextRange, TextSize};
 pub use crate::{
     cancellation::Canceled,
     input::{
-        CrateGraph, CrateId, CrateName, Dependency, Edition, Env, ExternSource, ExternSourceId,
-        FileId, ProcMacroId, SourceRoot, SourceRootId,
+        CrateData, CrateGraph, CrateId, CrateName, Dependency, Edition, Env, ExternSource,
+        ExternSourceId, FileId, ProcMacroId, SourceRoot, SourceRootId,
     },
 };
 pub use relative_path::{RelativePath, RelativePathBuf};