[5.6] Fix MorphTo lazy eager loading

[staudenmeir]

Collection::load() uses the first model to get the relationships for lazy eager loading. This can cause problems on MorphTo relationships when the related models differ in certain aspects:

Example 1: Different relationships in $with (#16604)

class Comment extends Model {
    public function commentable() {
        return $this->morphTo();
    }
}

class Post extends Model {
    protected $with = ['user'];

    public function user() {
        return $this->belongsTo(User::class);
    }
}

class Video extends Model {}

$post = Post::create(['user_id' => 1]);
$video = Video::create();
(new Comment)->commentable()->associate($post)->save();
(new Comment)->commentable()->associate($video)->save();

Comment::all()->load('commentable');

This fails because Eloquent tries to load the user relationship on $video (as specified in $post).

Example 2: Different primary keys (#24654)

class Comment extends Model {
    public function commentable() {
        return $this->morphTo();
    }
}

class Post extends Model {
    protected $primaryKey = 'post_id';
}

class Video extends Model {
    protected $primaryKey = 'video_id';
}

$post = Post::create();
$video = Video::create();
(new Comment)->commentable()->associate($post)->save();
(new Comment)->commentable()->associate($video)->save();

Comment::all()->load('commentable');

This fails because Eloquent tries to access the videos.post_id column (as specified in $post).

---

All this is caused by HasRelationships::morphTo() only calling morphEagerTo() on eager loading: On lazy eager loading, morphInstanceTo() is used.

We can fix the problem by always using a fresh model instance when getting the relationship in Builder::getRelation(). This mimics eager loading where the query builder is initialized with a fresh instance.

The new integration test combines different primary keys and the usage of $with.

Fixes #16604 and fixes #24654.