pkg/http_server: move to multi-lib layout

pkg/http_server/lib/src/virtual_directory.dart

Index: pkg/http_server/lib/src/virtual_directory.dart
diff --git a/pkg/http_server/lib/src/virtual_directory.dart b/pkg/http_server/lib/src/virtual_directory.dart
index d82fd992c6e30e6bc287f83b15920e3a49e3646e..bd8ca73ef845b2cc3d133f6c1a26ffa904e6007b 100644
--- a/pkg/http_server/lib/src/virtual_directory.dart
+++ b/pkg/http_server/lib/src/virtual_directory.dart
@@ -2,8 +2,14 @@
 // 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.
 
-part of http_server;
+library http_server.virtual_directory;
 
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:mime/mime.dart';
+import 'package:path/path.dart';
 
 // Used for signal a directory redirecting, where a tailing slash is missing.
 class _DirectoryRedirect {