_install(label, type) {
    this._removeLinks(label, type);
    commands[type].invoke('install', { cwd: this.dirs[label] });
    this._restoreLinks(label, type);

    // If we just did a clean install we can treat it as up-to-date.
    upgraded[label] = true;
  }
The `_install` method installs the contents of the manifest into the
specified package cache.

@private
@method _install
@param {String} label The label for the cache.
@param {String} type The type of package cache.