Introduce importlib.abc. The module contains various ABCs related to imports

(mostly stuff specified by PEP 302). There are two ABCs, PyLoader and
PyPycLoader, which help with implementing source and source/bytecode loaders by
implementing load_module in terms of other methods. This removes a lot of
gritty details loaders typically have to worry about.

Lib/importlib/test/abc.py

@@ -65,10 +65,11 @@ class LoaderTests(unittest.TestCase, metaclass=abc.ABCMeta):
 
         Attributes to verify:
 
-            * __file__
-            * __loader__
             * __name__
+            * __file__
+            * __package__
             * __path__
+            * __loader__
 
         """
         pass