[5.7] Fix HasManyThrough existence queries with same parent and through parent table

[staudenmeir]

Laravel already supports self-referencing HasManyThrough relationships with the same parent and related model (#26662).

This PR adds support for relationships with the same parent and through parent model:

class Category extends Model {
    public function subProducts() {
        return $this->hasManyThrough(Product::class, self::class, 'parent_id');
    }
}

Category::has('subProducts')->get();

In existence/withCount() queries, we have to alias the through parent table:

# expected
select * from `categories` where exists (
  select * from `products`
  inner join `categories` as `laravel_reserved_0`
  on `laravel_reserved_0`.`id` = `products`.`category_id`
  where `categories`.`id` = `laravel_reserved_0`.`parent_id`
)

# actual
select * from `categories` where exists (
  select * from `products`
  inner join `categories`
  on `categories`.`id` = `products`.`category_id`
  where `categories`.`id` = `categories`.`parent_id`
)

Fixes #26623.