[9.x] Fix base query for relationships

[driesvints]

This PR fixes an issue where scopes (like soft deletes) weren't applied when the base query of a relationship was retrieved. Previously, in Laravel v8, this was still applied properly.

Fixes #41885

[taylorotwell]

We need to dig into why this method was changed for Laravel 9.x - there was likely a reason.

[tpetry]

The toBase method was not implemented in Relation for Laravel 8. As of the query builder's mixin and forwarding principle the method call was forwarded by __call() to Eloquent\Builder::toBase() which is executing $this->applyScopes()->getQuery(); - the same as this PR.

So, my guess is that this is not an intended change but simply an oversight in the implementation because with the query builder interface the toBase method needed to be implemented.

[tpetry]

@taylorotwell @driesvints In my opinion is the now merged fix is adding a new inconsistency.

In Laravel v8 the implementation for getBaseQuery was:

public function getBaseQuery()
{
    return $this->query->getQuery();
}

With the new fix it's now the following which is different (it's now applying the scope!):

public function getBaseQuery()
{
    return $this->toBase();
}

public function toBase()
{
    return $this->query->applyScopes()->getQuery();
}

[driesvints]

@tpetry ah right, can you send in a PR for that?

[tpetry]

Give me some minutes 😉