Move completion APIs to a private location

pkg/analysis_server/lib/src/services/completion/completion_manager.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.completion.manager;
 
 import 'dart:async';
 
-import 'package:analysis_server/completion/completion_core.dart'
-    show CompletionRequest, CompletionResult;
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/protocol.dart';
+import 'package:analysis_server/src/provisional/completion/completion_core.dart'
+    show CompletionRequest, CompletionResult;
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/search/search_engine.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * [CompletionCache] contains information about the prior code completion
  * for use in the next code completion.
  */
@@ skipping 274 matching lines @@
    */
   final String name;
 
   /**
    * The elapse time or `null` if undefined.
    */
   final Duration elapsed;
 
   OperationPerformance(this.name, this.elapsed);
 }