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/distribution-types/src/annotation.rs

@@ -22,17 +22,17 @@ impl std::fmt::Display for SourceAnnotation {
         match self {
             Self::Requirement(origin) => match origin {
                 RequirementOrigin::File(path) => {
-                    write!(f, "-r {}", path.user_display())
+                    write!(f, "-r {}", path.portable_display())
                 }
                 RequirementOrigin::Project(path, project_name) => {
-                    write!(f, "{project_name} ({})", path.user_display())
+                    write!(f, "{project_name} ({})", path.portable_display())
                 }
             },
             Self::Constraint(origin) => {
-                write!(f, "-c {}", origin.path().user_display())
+                write!(f, "-c {}", origin.path().portable_display())
             }
             Self::Override(origin) => {
-                write!(f, "--override {}", origin.path().user_display())
+                write!(f, "--override {}", origin.path().portable_display())
             }
         }
     }