Fix rustdoc source code rendering for `#[path = "../path/to/mod.rs"]` links
Fixes#103517
While generating the location for modules source HTML to be saved at, a `..` path component appeared to be translated to `/up/`.
Additionally, while generating the navigation sidebar, `..` path components were ignored. This means that (as in the issue above), a *real* directory structure of:
```
sys/
unix/
mod.rs <-- contains #![path = "../unix/mod.rs]
cmath.rs
```
was rendered as:
```
sys/
unix/
mod.rs
unix/
cmath.rs <-- links to sys/unix/unix/cmath.rs.html, 404
```
While the *files* were stored as
```
sys/
unix/
mod.rs.html
up/
unix/
cmath.rs.html
```
eb27b613b6