[eggjs/egg]egg 不支持 Yarn 2

egg 在 Yarn 2 下无法运行

https://github.com/duanzheng/test-egg-yarn-v2

1. 安装最新版的 Yarn，并按照官方教程开启 Yarn 2 模式
2. 在项目根目录执行 yarn install
3. 执行 yarn dev，此时会报错

   Error: egg is not found in /Users/tony/Documents/myCode/egg-test/node_modules
   at assertAndReturn (/Users/tony/Documents/myCode/egg-test/.yarn/cache/egg-utils-npm-2.4.1-a2af25016f-2.zip/node_modules/egg-utils/lib/framework.js:66:9)
   at Object.getFrameworkPath (/Users/tony/Documents/myCode/egg-test/.yarn/cache/egg-utils-npm-2.4.1-a2af25016f-2.zip/node_modules/egg-utils/lib/framework.js:49:10)
   at DevCommand.formatArgs (/Users/tony/Documents/myCode/egg-test/.yarn/cache/egg-bin-npm-4.14.1-b4aed5d5cd-2.zip/node_modules/egg-bin/lib/cmd/dev.js:89:28)
   at formatArgs.next (<anonymous>)
   at onFulfilled (/Users/tony/Documents/myCode/egg-test/.yarn/cache/co-npm-4.6.0-dd9caee4b9-2.zip/node_modules/co/index.js:65:19)
   at /Users/tony/Documents/myCode/egg-test/.yarn/cache/co-npm-4.6.0-dd9caee4b9-2.zip/node_modules/co/index.js:54:5
   at new Promise (<anonymous>)
   at co (/Users/tony/Documents/myCode/egg-test/.yarn/cache/co-npm-4.6.0-dd9caee4b9-2.zip/node_modules/co/index.js:50:10)
   at toPromise (/Users/tony/Documents/myCode/egg-test/.yarn/cache/co-npm-4.6.0-dd9caee4b9-2.zip/node_modules/co/index.js:118:63)
   at next (/Users/tony/Documents/myCode/egg-test/.yarn/cache/co-npm-4.6.0-dd9caee4b9-2.zip/node_modules/co/index.js:99:29)

看了一下报错的 egg-utils/lib/framework.js 的代码，代码中写死了会去找 node_modules 中的文件，然而 Yarn 2 默认是没有 node_modules 的，这个也是我们需要的新特性。

• 操作系统：macOS 10.15.4
• Node 版本：12.16.2
• Egg 版本：2.15.1