You can't switch the internal (Lucene) search to index by URL. If you want this type of functionality you will need to use Bing or Google. To use Bing or Google, go to Administration > Site Settings > API Keys (tab) and select your desired search engine from the "Default Site Search Engine" drop down. To use either Google or Bing, you'll need to register for a "Custom Search Id" or "Search API Key," respectively, and place that value in the appropriate field on the API Keys page.
HTH,
Joe D.