Prevent users from shadowing builtins.pyi in ptt

Summary:
When using buck-based per-target type checking, we realised that
it is possible for users, by setting `base_module = ''` and having
an `__init__.py`, to shadow `builtins.py`.

This is pretty confusing because there's no user-visible indication
that this is happening, and we just get tons of type errors on all
the builtin types being undefined.

Reviewed By: grievejia

Differential Revision: D65076559

fbshipit-source-id: d1ea05c3d0b5c6a8b2e114ba42fb7b81e2f1f1a8

source/buck_command/buckBasedSourceCodeApi.ml

@@ -78,9 +78,17 @@ let create
   let source_index = build_index ~configuration:dummy_configuration source_module_paths in
   let dependency_index = build_index ~configuration:dummy_configuration dependency_module_paths in
   let merged_index =
-    (* Source files always take priority over dependency files *)
-    (* TODO: Revisit this decision once we get more results from production *)
-    let combine ~key:_ source_value _dependency_value = source_value in
+    (* Source files always take priority over dependency files, except in the special case of the
+       Reference.empty module name, in which case we make sure that it is not possible for an
+       `__init__.py` to shadow `builtins.pyi`. *)
+    let combine ~key:_ source_module_path dependency_module_path =
+      match
+        ( String.equal (ModulePath.relative dependency_module_path) "builtins.pyi",
+          String.equal (ModulePath.relative source_module_path) "builtins.pyi" )
+      with
+      | true, false -> dependency_module_path
+      | _, _ -> source_module_path
+    in
     Map.merge_skewed source_index dependency_index ~combine
   in
   let implicit_modules =

source/buck_command/test/sourceCodeApiTest.ml

@@ -186,7 +186,7 @@ let test_source_code_api =
       in
       create loader listing
     in
-    assert_lookup_relative_path ~context ~api [!"", Some "__init__.py"];
+    assert_lookup_relative_path ~context ~api [!"", Some "builtins.pyi"];
     ());
   ]