Move importlib's built-in importer to use rpartition for __package__.

Lib/importlib/_bootstrap.py

@@ -115,7 +115,9 @@ class BuiltinImporter:
         """Load a built-in module."""
         if fullname not in sys.builtin_module_names:
             raise ImportError("{0} is not a built-in module".format(fullname))
-        return imp.init_builtin(fullname)
+        module = imp.init_builtin(fullname)
+        module.__package__ = ''
+        return module
 
 
 class FrozenImporter:

Lib/importlib/test/builtin/test_loader.py

@@ -15,7 +15,7 @@ class LoaderTests(abc.LoaderTests):
     assert 'errno' in sys.builtin_module_names
     name = 'errno'
 
-    verification = {'__name__': 'errno', '__package__': None}
+    verification = {'__name__': 'errno', '__package__': ''}
 
     def verify(self, module):
         """Verify that the module matches against what it should have."""