Use a cross-platform representation for relative paths in pip compile

[charliermarsh]

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 #3800.

[zanieb]

Cool if it works!

[charliermarsh]

Famous last words.

[codspeed-hq]

CodSpeed Performance Report

Merging #3804 will not alter performance

_{Comparing charlie/rel (0183c34) with main (dd27f2f)}

Summary

✅ 13 untouched benchmarks

[charliermarsh]

Alright, this isn't quite what we want. But I think I can use this crate to solve this specific problem.

[charliermarsh]

Specifically, I think I can use this when we .join.

[charliermarsh]

Ok, this definitely is not doing what I want.

[charliermarsh]

Okay, this actually does do what we want.