populate from one path to a different path

On populating documents, it would be great if we could assign the populated result to a different field, akin to how `$lookup` works.
```js
const post = await Post.findOne().populate({ path: 'userId', as: 'user' });
```

Start with Mongoose's populate API and the `$lookup` behavior referenced in the issue, then locate the existing populate implementation and related tests. Done means the requested `userId` population can expose its result through the separate `user` field, with behavior and edge cases covered by tests.

Written by the indexing model from the issue text.