Move completion APIs to a private location

pkg/analysis_server/lib/src/provisional/completion/completion_core.dart

 // Copyright (c) 2015, 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 analysis_server.completion.completion_core;
+library analysis_server.src.provisional.completion.completion_core;
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 /**
  * An object used to produce completions for a specific error. Completion
  * contributors are long-lived objects and must not retain any state between
  * invocations of [computeSuggestions].
@@ skipping 59 matching lines @@
    * the replacementOffset will be the offset of the beginning of the
    * identifier.
    */
   int get replacementOffset;
 
   /**
    * Return the list of suggestions being contributed by the contributor.
    */
   List<CompletionSuggestion> get suggestions;
 }