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

tests/html/css_rule_list_test.dart

-#library('CSSRuleListTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library CSSRuleListTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
 
   var isCSSRuleList =
       predicate((x) => x is List<CSSRule>, 'is a List<CSSRule>');
 
   useHtmlConfiguration();
 
   test("ClientRectList test", () {
     var sheet = document.styleSheets[0];
     List<CSSRule> rulesList = sheet.cssRules;
     expect(rulesList, isCSSRuleList);
   });
 }