_readManifest(label, type) {
    let readManifestDir = this.dirs[label];

    if (!readManifestDir) {
      return null;
    }

    let inputPath = path.join(readManifestDir, translate(type, 'manifest'));

    let result = null;
    try {
      result = fs.readFileSync(inputPath, 'utf8');
    } catch (error) {
      // Swallow non-exceptional errors.
      if (error.code !== 'ENOENT') {
        throw error;
      }
    }
    return result;
  }
The `_readManifest` method reads the on-disk manifest for the current
cache and returns its value.

@private
@method _readManifest
@param {String} label The label for the cache.
@param {String} type The type of package cache.
@return {String} The manifest file contents on disk.