rustdoc reports diagnostics against very/../long/PATH.md instead of normalizing to PATH.md

[fasterthanlime]

The same repro as #130470 can be used here: https://github.com/fasterthanlime/readme-md-error-reporting

When running cargo +stage1 t, the output is:

---- src/../README.md - (line 5) stdout ----
error: expected `;`, found `}`
 --> src/../README.md:7:16
  |
3 |     let x = 234 // no semicolon here! oh no!
  |                ^ help: add `;` here
4 | }
  | - unexpected token

But I would argue it should be this:

---- README.md - (line 5) stdout ----
error: expected `;`, found `}`
 --> README.md:7:16
  |
3 |     let x = 234 // no semicolon here! oh no!
  |                ^ help: add `;` here
4 | }
  | - unexpected token

I would caution against using canonicalize — which resolves symbolic links etc. and requires the path to exist on disk right now, and rather advise that y'all simply have a state machine working on a stack of path elements, going through:

• []
• ["src"]
• [] // just popped due to ".."
• ["README.md"]

[notriddle]

I can reproduce this problem without involving rustdoc.

$ mkdir -p check-dir/subdir
$ cat > check-dir/bar.rs
fn unusual() {}
$ cat > check-dir/subdir/foo.rs
include! { "../bar.rs" }
$ rustc +nightly --crate-type=lib check-dir/subdir/foo.rs
 --> check-dir/subdir/../bar.rs:1:4
  |
1 | fn unusual() {}
  |    ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: 1 warning emitted

[GuillaumeGomez]

I still wrote a little path beautifier. ^^'