demandimport: patch __builtin__ instead of __builtins__

This helps on implementations other than CPython, where __builtins__ isn't
necessarily defined (as it's an implementation detail for CPython).

mercurial/demandimport.py

@@ -24,6 +24,7 b' These imports will not be delayed:'
   b = __import__(a)
 '''
 
+import __builtin__
 _origimport = __import__
 
 class _demandmod(object):
@@ -126,9 +127,9 b' ignore = ['
 
 def enable():
     "enable global demand-loading of modules"
-    __builtins__["__import__"] = _demandimport
+    __builtin__.__import__ = _demandimport
 
 def disable():
     "disable global demand-loading of modules"
-    __builtins__["__import__"] = _origimport
+    __builtin__.__import__ = _origimport