[red-knot] Avoid including __all__ in the exported names

crates/red_knot_python_semantic/resources/mdtest/import/star.md

@@ -923,6 +923,9 @@ reveal_type(X)  # revealed: bool
 
 # TODO: should emit [unresolved-reference] diagnostic & reveal `Unknown`
 reveal_type(Y)  # revealed: bool
+
+# error: [unresolved-reference]
+reveal_type(__all__)  # revealed: Unknown
 ```
 
 ### `__all__` with additions later on in the global scope

crates/red_knot_python_semantic/src/semantic_index/re_exports.rs

@@ -71,10 +71,10 @@ impl<'db> ExportFinder<'db> {
     }
 
     fn possibly_add_export(&mut self, export: &'db Name, kind: PossibleExportKind) {
-        self.exports.insert(export, kind);
-
         if export == "__all__" {
             self.dunder_all = DunderAll::Present;
+        } else {
+            self.exports.insert(export, kind);
         }
     }