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

extensions/common/api/cast_channel/cast_channel.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;
+package extensions.api.cast_channel;
 
 message CastMessage {
   // Always pass a version of the protocol for future compatibility
   // requirements.
   enum ProtocolVersion {
     CASTV2_1_0 = 0;
   }
   required ProtocolVersion protocol_version = 1;
 
   // source and destination ids identify the origin and destination of the
@@ skipping 62 matching lines @@
   required ErrorType error_type = 1;
 }
 
 message DeviceAuthMessage {
   // Request fields
   optional AuthChallenge challenge = 1;
   // Response fields
   optional AuthResponse response = 2;
   optional AuthError error = 3;
 }