code: function resolveTargetPath(targetName, engine) {
      var dir = engine.options.dir || process.pwd();
      var ext = engine.options.extname || '.etpl';
  
      return path.resolve(dir, targetName) + ext;
  }
docs: 获取 target 对应的文件路径
  
  @inner
  @param {string} targetName target名称
  @param {Engine} engine etpl引擎
  @return {string}