[8.x] Add Eloquent builder whereMorphedTo method to streamline finding models morphed to another model

[tobyzerner]

This PR adds whereMorphedTo and orWhereMorphedTo methods to the Eloquent Builder.

These methods are a shortcut for adding a where condition looking for models that are morphed to a specific related model, without the overhead of a whereHas subquery.

As an example, consider the following model:

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

Say we want to find feedback which is about a specific $model. One way to do this is to use whereHas:

$feedback = Feedback::whereHas('subject', [$model->getMorphClass()], function ($query) use ($model) {
    $query->where('id', $model);
})->get();

However, this is verbose and adds an unnecessary subquery. The information we want to constrain against is already in the feedback table. So, a more performant way would be to do this:

$feedback = Feedback::query()
    ->where('subject_type', $model->getMorphClass())
    ->where('subject_id', $model->getKey())
    ->get();

This is better, but it's still rather verbose. Also, the subject_type and subject_id columns are hard-coded – it would be better if we didn't have to worry about them and could automatically use the column names defined on the MorphTo relationship itself.

With this PR, the above can be replaced by this:

$feedback = Feedback::whereMorphedTo('subject', $model)->get();

It adds the same where clause, but is much simpler to read/write, and uses the column names defined on the MorphTo relationship.

[taylorotwell]

I think it may be useful to be able to pass a string as the model name as well? ->whereMorphedTo('subject', Feedback::class);

[tobyzerner]

Done. Not sure why the tests are failing in that particular instance, but it doesn't seem to have to do with the PR.

[GrahamCampbell]

Isn't this wrong if the relation uses something other than the model key as the owner key? $model->getKey() should be $relation->getRelatedKeyFrom($model)?