[Bug] yarn add --peer after --dev doesn't add peer dependency

- [ ] I'd be willing to implement a fix



**Describe the bug**



I have a library that depends on `rxjs` as a peer dependency, but it also has tests inside it. 



If I add `rxjs` as a normal dependency, tests run properly. If instead I run:



`yarn add --peer --dev rxjs`



My tests will complain about: `Cannot find module 'rxjs' from '...'`



Note that I'm also using typescript and `ts-jest`. 



Ideally, the dependency should be available to both peer and dev dependencies.



**To Reproduce**



```js repro

const {promises: {readFile}} = require(`fs`);



await packageJsonAndInstall({

  devDependencies: {},

});



await yarn(`add`, `--dev`, `rxjs`);

await yarn(`add`, `--peer`, `rxjs`);



const pkgJson = JSON.parse(await readFile(`package.json`, `utf8`));

expect(pkgJson).toHaveProperty(`peerDependencies`);

expect(pkgJson).toHaveProperty(`devDependencies`);

expect(pkgJson.peerDependencies).toHaveProperty(`rxjs`);

expect(pkgJson.devDependencies).toHaveProperty(`rxjs`);

```



**Additional context**



This is also a bug in yarn v1: https://github.com/yarnpkg/yarn/issues/5287

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