Split out `RelPath` into a separate crate (#61029)

This is necessary to remove some `util` dependencies from crates, as
well as better sharing for our projects. This also includes the WIP
AbsPath abstraction as well as some bug fixes from internal tooling.


Release Notes:

- N/A or Added/Fixed/Improved ...
f181a2f47bLukas Wirth committed on 7/15/2026, 8:33:25 AM· committed by GitHubparentfebe242
64 files changedLine totals unavailable