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-interpreter/src/discovery.rs

@@ -1256,17 +1256,11 @@ impl fmt::Display for InterpreterNotFound {
                 path.user_display()
             ),
             Self::ExecutableNotFoundInDirectory(directory, executable) => {
-                let executable = if let Ok(relative_executable) = executable.strip_prefix(directory)
-                {
-                    relative_executable.display()
-                } else {
-                    executable.user_display()
-                };
                 write!(
                     f,
                     "Interpreter directory `{}` does not contain Python executable at `{}`",
                     directory.user_display(),
-                    executable
+                    executable.user_display_from(directory)
                 )
             }
             Self::ExecutableNotFoundInSearchPath(name) => {