Do not use relative path in rustc doc

[mu001999]

We have some relative links in doc, e.g., the link of qpath_res in the comments for QPath:

/// Represents an optionally `Self`-qualified value/type path or associated extension.
///
/// To resolve the path to a `DefId`, call [`qpath_res`].
///
/// [`qpath_res`]: ../../rustc_middle/ty/struct.TypeckResults.html#method.qpath_res
#[derive(Debug, Clone, Copy, HashStable_Generic)]
pub enum QPath<'hir> { ... }

but we will get the Not Found page if we click it in https://doc.rust-lang.org/stable/nightly-rustc/rustc_hir/enum.QPath.html, which is re-exported.

So I think we can fix this by two ways:

1. Don't use relative path in rustc doc
2. Don't generate new link for re-exported items, use the same links with the original definition

but for the second option, we may have private middle mods, so it may be not a solution.

[fmease]

cc #74481

[m1sk9]

I'm thinking of tackling this issue, is this Issue aimed at removing this invalid path once and for all?
(Sorry if I'm wrong as I'm trying to contribute for the first time)

[DanielFrantes]

@rustbot claim

[heiseish]

Hi @danik292, just checking if you are still working on the issue? If so, apologies please ignore this. Otherwise I'm happy to take over. Thanks

[DanielFrantes]

@saethlin take it