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/pkg/event/BUILD.bazel b/metropolis/pkg/event/BUILD.bazel
index 4e3b33c..0217c8a 100644
--- a/metropolis/pkg/event/BUILD.bazel
+++ b/metropolis/pkg/event/BUILD.bazel
@@ -1,20 +1,8 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
 
 go_library(
     name = "go_default_library",
-    srcs = [
-        "event.go",
-        "memory_value.go",
-    ],
+    srcs = ["event.go"],
     importpath = "source.monogon.dev/metropolis/pkg/event",
     visibility = ["//visibility:public"],
 )
-
-go_test(
-    name = "go_default_test",
-    srcs = [
-        "example_test.go",
-        "memory_value_test.go",
-    ],
-    embed = [":go_default_library"],
-)