Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

tests/html/fileapi_test.dart

-#library('fileapi');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library fileapi;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 void fail(message) {
   guardAsync(() {
     expect(false, isTrue, reason: message);
   });
 }
 
 DOMFileSystem fs;
 
 main() {
@@ skipping 55 matching lines @@
           successCallback: expectAsync1((FileEntry e) {
             expect(e.name, equals('file4'));
             expect(e.isFile, isTrue);
           }),
           errorCallback: (e) {
             fail('Got file error: ${e.code}');
           });
       });
   });
 }