code: function calcChunkHash(chunk) {
      var hash = '';
  
      var chunkModulesPath = chunk.mapModules(function(m) {
          return path.relative(m.context, m.request);
      }).join('_');
  
      hash = createHash(options.hashFunction).update(chunkModulesPath)
                                             .digest(options.hashDigest);
  
      // 截取 hash 的长度
      var len = options.hashDigestLength;
      while(usedHash.has(hash.substr(0, len))) {
          len++;
      }
      hash = hash.substr(0, len);
      usedHash.add(hash);
  
      return hash;
  }
docs: 计算 chunk 的 hash 值
  
  参考
  https://medium.com/webpack/predictable-long-term-caching-with-webpack-d3eee1d3fa31
  HashedModuleIdsPlugin
  
  @param {object} chunk
  @return {string}