getOfflineMirrorPath(packageFilename) {
    let mirrorPath;

    var _arr = ['npm', 'yarn'];
    for (var _i5 = 0; _i5 < _arr.length; _i5++) {
      const key = _arr[_i5];
      const registry = this.registries[key];

      if (registry == null) {
        continue;
      }

      const registryMirrorPath = registry.config['yarn-offline-mirror'];

      if (registryMirrorPath === false) {
        return null;
      }

      if (registryMirrorPath == null) {
        continue;
      }

      mirrorPath = registryMirrorPath;
    }

    if (mirrorPath == null) {
      return null;
    }

    if (packageFilename == null) {
      return mirrorPath;
    }

    return path.join(mirrorPath, path.basename(packageFilename));
  }
Remote packages may be cached in a file system to be available for offline installation.
Second time the same package needs to be installed it will be loaded from there.
Given a package's filename, return a path in the offline mirror location.