[ty] Implement stdlib stub mapping (#19529)

by using essentially the same logic for system site-packages, on the
assumption that system site-packages are always a subdir of the stdlib
we were looking for.

crates/ty_test/src/lib.rs

@@ -283,6 +283,7 @@ fn run_test(
             extra_paths: configuration.extra_paths().unwrap_or_default().to_vec(),
             custom_typeshed: custom_typeshed_path.map(SystemPath::to_path_buf),
             site_packages_paths,
+            real_stdlib_path: None,
         }
         .to_search_paths(db.system(), db.vendored())
         .expect("Failed to resolve search path settings"),