Clean up RelPath API (#38912)

Consolidate constructors and accessors.

Release Notes:

- N/A

---------

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

crates/remote_server/src/remote_editing_tests.rs

@@ -1039,7 +1039,7 @@ async fn test_adding_then_removing_then_adding_worktrees(
         assert!(worktree.is_visible());
         let entries = worktree.entries(true, 0).collect::<Vec<_>>();
         assert_eq!(entries.len(), 2);
-        assert_eq!(entries[1].path.as_str(), "README.md")
+        assert_eq!(entries[1].path.as_unix_str(), "README.md")
     })
 }
 

crates/remote_server/src/unix.rs

@@ -953,7 +953,7 @@ fn cleanup_old_binaries() -> Result<()> {
     let release_channel = release_channel::RELEASE_CHANNEL.dev_name();
     let prefix = format!("zed-remote-server-{}-", release_channel);
 
-    for entry in std::fs::read_dir(server_dir)? {
+    for entry in std::fs::read_dir(server_dir.as_std_path())? {
         let path = entry?.path();
 
         if let Some(file_name) = path.file_name()