Use a cross-platform representation for relative paths in pip compile (#3804)

## Summary

I haven't tested on Windows yet, but the idea here is that we should use
a portable representation when printing paths.

I decided to limit the scope here to paths that we write to output
files.

Closes https://github.com/astral-sh/uv/issues/3800.

crates/uv-configuration/src/overrides.rs

@@ -1,6 +1,6 @@
 use std::hash::BuildHasherDefault;
 
-use itertools::Either;
+use either::Either;
 use rustc_hash::FxHashMap;
 
 use distribution_types::Requirement;