Clean up RelPath API (#38912)

Consolidate constructors and accessors.

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/languages/src/python.rs

@@ -55,7 +55,7 @@ impl ManifestProvider for PyprojectTomlManifestProvider {
         }: ManifestQuery,
     ) -> Option<Arc<RelPath>> {
         for path in path.ancestors().take(depth) {
-            let p = path.join(RelPath::new("pyproject.toml").unwrap());
+            let p = path.join(RelPath::unix("pyproject.toml").unwrap());
             if delegate.exists(&p, Some(false)) {
                 return Some(path.into());
             }
@@ -1030,7 +1030,7 @@ impl ToolchainLister for PythonToolchainProvider {
         config.workspace_directories = Some(
             subroot_relative_path
                 .ancestors()
-                .map(|ancestor| worktree_root.join(ancestor))
+                .map(|ancestor| worktree_root.join(ancestor.as_std_path()))
                 .collect(),
         );
         for locator in locators.iter() {