optimize lookup_special_export

Summary:
Profiling the indexing of langchain/dify I found >25% of the time was spent waiting on locks in lookup_special_export

There's lots of contention for calculating exports here since we check lookup_special_export so much in bindings

Longer AI explanation: P2152790830

Adding a fast return path that avoids pulling exports before entering the loop reduces the contention.

Reviewed By: rchen152

Differential Revision: D91753332

fbshipit-source-id: f89ef539fa5c9ca8fcc495806969d27b137062b6

Author: yangdanny97

pyrefly/lib/binding/scope.rs

@@ -1761,6 +1761,12 @@ impl Scopes {
         mut module: ModuleName,
         lookup: &dyn LookupExport,
     ) -> Option<SpecialExport> {
+        // Fast check to avoid calculating exports, reducing contention
+        if let Some(special) = SpecialExport::new(&name)
+            && special.defined_in(module)
+        {
+            return Some(special);
+        }
         let mut seen = HashSet::new();
         let mut exports = lookup.get(module).finding()?.exports(lookup);
         loop {