[5.5] Fix eager loading HasManyThrough relations with custom intermediate and local key

[druc]

Hello,

Whenever I try to eager load a hasManyThrough relation that uses a custom intermediate and a local key I get an empty collection.

This happens because when the match(array $models, Collection $results, $relation) on the HasManyThrough class is called, the relation is set using the $model->getKey rather than my provided $this->localKey attribute.

Changing

foreach ($models as $model) {
    if (isset($dictionary[$key = $model->getKey()])) {
        //
    }
}

into

foreach ($models as $model) {
    if (isset($dictionary[$key = $model->getAttribute($this->localKey)])) {
        //
    }
}

fixes it because $this->localKey defaults to the pk anyway.

Also, I think this is also a fix for this issue #15909 (haven't tested, but looks like the same problem)