Respect per-module follow_import for empty folders

Fixes #13757

Author: hauntsaninja

mypy/build.py

@@ -2592,7 +2592,7 @@ def find_module_and_diagnose(
         if (
             root_source  # Honor top-level modules
             or (
-                not result.endswith(".py")  # Stubs are always normal
+                result.endswith(".pyi")  # Stubs are always normal
                 and not options.follow_imports_for_stubs
             )  # except when they aren't
             or id in mypy.semanal_main.core_modules