m/pkg/event: move MemoryValue to subpackage

This keeps metropolis/pkg/event as a pure interface package, and
moves the memory-backed implementation to a subpackage.

Test Plan: Refactor, coevered by tests.

X-Origin-Diff: phab/D764
GitOrigin-RevId: 1337bf55a7752293791b3efe8648bbf5f6e6e9e1
diff --git a/metropolis/node/core/cluster/manager.go b/metropolis/node/core/cluster/manager.go
index 874d3ae..eede40f 100644
--- a/metropolis/node/core/cluster/manager.go
+++ b/metropolis/node/core/cluster/manager.go
@@ -30,6 +30,7 @@
 	"source.monogon.dev/metropolis/node/core/localstorage"
 	"source.monogon.dev/metropolis/node/core/network"
 	"source.monogon.dev/metropolis/pkg/event"
+	"source.monogon.dev/metropolis/pkg/event/memory"
 	"source.monogon.dev/metropolis/pkg/supervisor"
 	apb "source.monogon.dev/metropolis/proto/api"
 	ppb "source.monogon.dev/metropolis/proto/private"
@@ -132,7 +133,7 @@
 type Manager struct {
 	storageRoot    *localstorage.Root
 	networkService *network.Service
-	status         event.MemoryValue
+	status         memory.Value
 
 	state