Issue #3230668: Split out Composer Stager wiring into Package Manager submodule

tests/src/Kernel/UpdaterTest.php

@@ -58,7 +58,7 @@ class UpdaterTest extends KernelTestBase {
       '--update-with-all-dependencies',
     ];
     $stager->stage($command, Argument::cetera())->shouldBeCalled();
-    $this->container->set('automatic_updates.stager', $stager->reveal());
+    $this->container->set('package_manager.stager', $stager->reveal());
     $this->container->get('automatic_updates.updater')->stage();
   }