Issue #2377: Make importlib the implementation of __import__().

importlib._bootstrap is now frozen into Python/importlib.h and stored
as _frozen_importlib in sys.modules. Py_Initialize() loads the frozen
code along with sys and imp and then uses _frozen_importlib._install()
to set builtins.__import__() w/ _frozen_importlib.__import__().

Lib/importlib/test/util.py

@@ -35,7 +35,7 @@ def uncache(*names):
     for name in names:
         if name in ('sys', 'marshal', 'imp'):
             raise ValueError(
-                "cannot uncache {0} as it will break _importlib".format(name))
+                "cannot uncache {0}".format(name))
         try:
             del sys.modules[name]
         except KeyError: