docs: @Exported
      public String getUrl() {
          // first look for the manifest entry. This is new in maven-hpi-plugin 1.30
          String url = manifest.getMainAttributes().getValue("Url");
          if(url!=null)      return url;
  
          // fallback to update center metadata
          UpdateSite.Plugin ui = getInfo();
          if(ui!=null)    return ui.wiki;
  
          return null;
      }