Provide an API to get imports, particularly `pub use`

[euclio]

This is needed for the new rustdoc. Right now it is possible to get defs that are exported through pub use, but we have no way to distinguish them from naturally exported items.

Ideally rustdoc would be able to get all defs and imports, and by looping through the imports, determine if the import is an export or a re-export of a given def.

What should the API for this look like?

[nrc]

What exactly does Rustdoc need to know here? How does it use the info and why does it need to distinguish between an export and a re-export?

[euclio]

The new rustdoc needs to be able to create links to a given Def at all paths that the Def is accessible through, including pub uses.

Currently, the new rustdoc works by recursively traversing the crate root Def and its children. Assuming that we are able to filter analysis data by reachability, it doesn't find any Defs that have a non-exported Def in their ancestry. However, these Defs might be exported through a pub use. rustdoc needs to link the Def at the path it's exported through. If a Def is accessible through multiple paths, rustdoc needs to indicate that those paths are re-exports of the other path.

I'm not sure that rls-analysis needs to distinguish between exports and re-exports (the client could probably infer it), but the new rustdoc certainly needs to display the two differently, like the current rustdoc does.

[euclio]

@nrc I realize that imports are already recorded in the analysis data, but I'm confused how to get the path that the import is at. I'm guessing rls-data/the compiler will have to be modified to include qualname or a similar field in an Import?

[nrc]

I would rather add a parent field than a qualname one - qualname has been irritating in the past and I think a parent gives the required data.

[euclio]

I'm going to close this, as the upstream project (rustdoc2) is dormant.