Use some canonical path format

[Xanewok]

Originally created by @staktrace (rust-dev-tools/rls-data#20):

Right now if running the rust compiler on Windows with -Zsave-analysis, the Span::file_name items usually end up with windows-style paths (e.g. Z:\foo\bar.txt). However in some cases they appear to have a mix of windows and unix dir separators (e.g. Z:\foo\bar/lib.rs). This is just weird. It should be consistent in what slashes it's using.

For context, this came up when trying analyze these files for searchfox on a linux host. The rls-analysis crate will read save-analysis files generated on Windows, but the Span::file_name ends up as a PathBuf of a single component because \ is a valid filename char on Linux. I can deal with this by doing a search-and-replace, but when the separators are mixed (both \ and /) it makes me uneasy.

/cc @emilio

[hh9527]

use relative-path crate, always "/" is a better idea, it will avoid many corner case.