[MINOR BC BREAK] Syncing: PivotWithRelation -> PivotWithCentralResource

The old names of the class and method were misleading. We don't
actually need any relation. And we don't even need a model instance
as we were returning previously -- the only use of that method was
in TriggerSyncingEvents which would immediately use ::class on the
returned value. Therefore, all we are asking for in this interface
is just the central resource class.

src/ResourceSyncing/CentralResourceNotAvailableInPivotException.php

@@ -13,7 +13,7 @@ class CentralResourceNotAvailableInPivotException extends Exception
         parent::__construct(
             'Central resource is not accessible in pivot model.
             To attach a resource to a tenant, use $centralResource->tenants()->attach($tenant) instead of $tenant->resources()->attach($centralResource) (same for detaching).
-            To make this work both ways, you can make your pivot implement PivotWithRelation and return the related model in getRelatedModel() or extend MorphPivot.'
+            To make this work both ways, you can make your pivot implement PivotWithCentralResource and return the related model in getCentralResourceClass() or extend MorphPivot.'
         );
     }
 }

src/ResourceSyncing/PivotWithCentralResource.php

@@ -0,0 +1,11 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stancl\Tenancy\ResourceSyncing;
+
+interface PivotWithCentralResource
+{
+    /** @return class-string<\Illuminate\Database\Eloquent\Model&Syncable> */
+    public function getCentralResourceClass(): string;
+}

src/ResourceSyncing/PivotWithRelation.php

@@ -1,15 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace Stancl\Tenancy\ResourceSyncing;
-
-use Illuminate\Database\Eloquent\Model;
-
-interface PivotWithRelation
-{
-    /**
-     * E.g. return $this->users()->getModel().
-     */
-    public function getRelatedModel(): Model;
-}

src/ResourceSyncing/TriggerSyncingEvents.php

@@ -79,9 +79,9 @@ trait TriggerSyncingEvents
      */
     protected function getResourceClass(): string
     {
-        /** @var $this&(Pivot|MorphPivot|((Pivot|MorphPivot)&PivotWithRelation)) $this */
-        if ($this instanceof PivotWithRelation) {
-            return $this->getRelatedModel()::class;
+        /** @var $this&(Pivot|MorphPivot|((Pivot|MorphPivot)&PivotWithCentralResource)) $this */
+        if ($this instanceof PivotWithCentralResource) {
+            return $this->getCentralResourceClass();
         }
 
         if ($this instanceof MorphPivot) {