can't get plugin rules by `CLIEngine#getRules`

**Tell us about your environment**

* **ESLint Version:**
6.0.0

`cli.getRules().get(ruleId)` used can get plugin rule, now not.

Is this by design or a bug?



```js
new (require('eslint').CLIEngine)().getRules().has('yoda')
// -> true
```

```js
new (require('eslint').CLIEngine)({plugins: ['unicorn']}).getRules().has('unicorn/no-process-exit')
// -> false, should be true
```