code: function CommonJsProject(opts) {
    this.roots = opts.roots;
    this.textPluginPattern = opts.textPluginPattern || /^text!/;
  
    opts.roots = this.roots.map(function(root) {
      if (!copy.isDirectory(root)) {
        throw new Error('Each commonjs root should be a directory: ' + root);
      }
      return ensureTrailingSlash(root);
    }, this);
  
    // A module is a Location that also has dep
    this.currentModules = {};
    this.ignoredModules = {};
  }
docs: Keep track of the files in a project