Generate all extension schema namespaces as "api" and instead vary the generated bundle names.

extensions/common/api/cast_channel/authority_keys.proto

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
-package extensions.core_api.cast_channel.proto;
+package extensions.api.cast_channel.proto;
 
 message AuthorityKeys {
   message Key {
     required bytes fingerprint = 1;
     required bytes public_key = 2;
   }
   repeated Key keys = 1;
 }