遵循Truffle的软件包管理文档：https://truffleframework.com/docs/truffle/getting-started/package-management-via-ethpm
在尝试安装软件包时：
truffle install ens
显示以下错误：
> Error: Cannot find any published versions of 'ens' with the specified
> version range (*)
> at /usr/local/lib/node_modules/truffle/build/webpack:/~/ethpm-registry/lib/registry.js:85:1
> at <anonymous>
> at process._tickCallback (internal/process/next_tick.js:188:7)
> Truffle v5.0.2 (core: 5.0.2)
> Node v8.11.2