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/memory/BUILD.bazel b/metropolis/pkg/event/memory/BUILD.bazel
new file mode 100644
index 0000000..4ba79d2
--- /dev/null
+++ b/metropolis/pkg/event/memory/BUILD.bazel
@@ -0,0 +1,19 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["memory.go"],
+    importpath = "source.monogon.dev/metropolis/pkg/event/memory",
+    visibility = ["//visibility:public"],
+    deps = ["//metropolis/pkg/event:go_default_library"],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = [
+        "example_test.go",
+        "memory_test.go",
+    ],
+    embed = [":go_default_library"],
+    deps = ["//metropolis/pkg/event:go_default_library"],
+)