feat(tvix/eval): context-aware baseNameOf

Change-Id: I9f0a8143070805b85276f721bdfbdf7ede2cf615
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10421
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -161,9 +161,12 @@ mod pure_builtins {
                     span,
                 )
                 .await?
-                .to_str()?,
+                .to_contextful_str()?,
         };
-        let result: String = s.rsplit_once('/').map(|(_, x)| x).unwrap_or(&s).into();
+        let result: NixString = NixString::new_inherit_context_from(
+            &s,
+            s.rsplit_once('/').map(|(_, x)| x).unwrap_or(&s),
+        );
         Ok(result.into())
     }