code: function subModulePath(subModule, node_modules) {
    var index = subModule.indexOf("/");
    var length = subModule.length;
    if (index <= 0 || index + 1 == length) {
      return null;
    }
    var module = subModule.substring(0, index);
    var relative = subModule.substring(index + 1, length);
  
    var moduleDirectory = path.join(node_modules, module);
    var filePath = path.join(moduleDirectory, relative);
    return realRequirPath(filePath);
  }
docs: get the full path of subModule
  
  @param subModule subModule
  @returns {*} /User/XXXX/node_modules/module/relative.js