Add `cargo fix` support for switching to intra-doc links

[jyn514]

This will be tricky for several reasons.

• It needs pulldown support to look at the links even if they're not broken. Event and Tag seem like they should work.
• Rustdoc needs to know what the link is meant to resolve to. This can be tricky if the item is re-exported in multiple places (see e.g. Broken links in String documentation from Deref impl #32129). Rustdoc should report all possible items you meant if there's ambiguity (and downgrade the applicability).
• Rustdoc needs to know what name in scope matches with the item you're linking too. This seems like it shouldn't be too hard, but I don't see a way to go from DefId -> Path on Resolver, only the reverse. cc @Manishearth, do you have ideas?

This can be implemented as an allowed-by-default lint with Applicability::MachineApplicable; then cargo fix will pick it up automatically.

[jyn514]

I don't see a way to go from DefId -> Path on Resolver

Maybe def_path_str would work for this. I'm worried that will only give the absolute path, not the relative path, though (see also #50310).

[Manishearth]

It's been a long standing bug that diagnostics should print out a local path, so, I don't think it's easy to do yet. The information exists, just not the code for it

[jyn514]

I mean, I don't mind trying to fix #50310 at the same time. Where's the information stored?

ScopeTree seems vaguely related.

[Manishearth]

Unsure, @eddyb may know