Querying related models make unnecessary requests

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## Steps to reproduce

Create a model called Book with the properties id,author,title
Add a hasMany relation between the default User model and the Book model using a through Model called UserToBook which has the following properties: userId,bookId

The relation is defined in the User model like so.
```json
"books": {
"type": "hasMany",
"model": "Book",
"foreignKey": "userId",
"through": "UserToBook",
"keyThrough": "bookId"
},
```

## Current Behavior

Querying the user for his books using a where filter performs 2 requests - one to the through model and one to the actual related model. The query to the through model is adding an `include` to the related model, which causes a fetch of a lot of data from the related model.
The reason this data is needed, is to extract the related model ids, so we can use it to filter out when performing the actual call the related model.

## Expected Behavior

There is no need at all to fetch the related model data in the first call - we can simply get the ids from the through model, and use them

## Link to reproduction sandbox

Working on it

## Additional information

Since I'm using (sadly) loopback 2.x I have added the fix to my own fork - https://github.com/regevbr/loopback-datasource-juggler/blob/2.x/lib/scope.js
You can understand and base the fix based on the changes there

https://github.com/regevbr/loopback-datasource-juggler/blob/2cf8269e41a153d01e4d505af586bad9bb9110dd/lib/scope.js#L100-L138

Start by reading lib/scope.js, especially the behavior shown in the linked commit and the reported 2.x fork. Reproduce the hasMany-through query with a where filter and verify that the through-model request returns only related IDs without fetching full related-model data, while the final related-model query still applies the filter.

Written by the indexing model from the issue text.