Menu entity should clear plugin.manager.block even when block module not installed

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Reviewed the changes and removing the moduleExist check looks good.

Ran the tests locally to make sure they fail and got

Failed asserting that an array has the key 'system_menu_block:another_menu'.

Which is expected.

Looks good!

If used a memory cache for the static cache, we could rely on cache tags here, cross-linking 🌱 [policy] Standardize how we implement in-memory caches Needs work .

One nit on the patch itself:

+++ b/core/modules/system/src/Plugin/Derivative/SystemMenuBlock.php
@@ -44,6 +44,8 @@ public static function create(ContainerInterface $container, $base_plugin_id) {
    * {@inheritdoc}
    */
   public function getDerivativeDefinitions($base_plugin_definition) {
+    // Reset the discovered definitions.
+    $this->derivatives = [];
     foreach ($this->menuStorage->loadMultiple() as $menu => $entity) {

This needs a better comment - why do we need to reset the property every time it's called?