Use portable slash paths in lockfile (#4324)

## Summary

This would be a lightweight solution to
https://github.com/astral-sh/uv/issues/4307 that doesn't fully engage
with all the possibilities in the design space (but would unblock
cross-platform for now).

Cargo.lock

@@ -4864,6 +4864,7 @@ dependencies = [
  "itertools 0.13.0",
  "once-map",
  "owo-colors",
+ "path-slash",
  "pep440_rs",
  "pep508_rs",
  "petgraph",

crates/uv-resolver/Cargo.toml

@@ -42,6 +42,7 @@ futures = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }
 owo-colors = { workspace = true }
+path-slash = { workspace = true }
 petgraph = { workspace = true }
 pubgrub = { workspace = true }
 rkyv = { workspace = true }

crates/uv-resolver/src/lock.rs

@@ -9,6 +9,7 @@ use std::path::{Path, PathBuf};
 use std::str::FromStr;
 
 use either::Either;
+use path_slash::PathExt;
 use petgraph::visit::EdgeRef;
 use rustc_hash::FxHashMap;
 use serde::{Deserialize, Deserializer};
@@ -851,8 +852,11 @@ enum Source {
 }
 
 /// A [`PathBuf`], but we show `.` instead of an empty path.
+///
+/// We also normalize backslashes to forward slashes on Windows, to ensure
+/// that the lock file contains portable paths.
 fn serialize_path_with_dot(path: &Path) -> Cow<str> {
-    let path = path.to_string_lossy();
+    let path = path.to_slash_lossy();
     if path.is_empty() {
         Cow::Borrowed(".")
     } else {

crates/uv/tests/lock.rs

@@ -2478,25 +2478,8 @@ fn relative_and_absolute_paths() -> Result<()> {
 
     let lock = fs_err::read_to_string(context.temp_dir.join("uv.lock"))?;
 
-    // In this particular test, Windows paths with \ are written to the TOML
-    // lock file, but because the \ starts an escape sequence in TOML strings,
-    // this causes the TOML serializer to use single quoted strings. But
-    // everywhere else, / is used and thus double quoted strings are used.
-    //
-    // Making matters more confusing, we have filters that normalize Windows
-    // paths to Unix paths, which makes it *look* like the TOML in the snapshot
-    // is needlessly using single quoted strings.
-    //
-    // So we add a filter here (that runs after all other filters) that
-    // replaces single quoted strings with double quoted strings. This isn't
-    // correct in general, but works for this specific test.
-    let filters = context
-        .filters()
-        .into_iter()
-        .chain(vec![(r"'([^']+)'", r#""$1""#)])
-        .collect::<Vec<_>>();
     insta::with_settings!({
-        filters => filters,
+        filters => context.filters(),
     }, {
         assert_snapshot!(
             lock, @r###"