metropolis: Lock down visibility rules

This formalizes the package structure introduced by D683.

Test Plan: Pure refactor, CI only.

X-Origin-Diff: phab/D684
GitOrigin-RevId: 574aa14c71faf94f4a5c02a2110e2e3fef7d36ac
diff --git a/metropolis/node/core/BUILD.bazel b/metropolis/node/core/BUILD.bazel
index 004bbc8..b9d9d03 100644
--- a/metropolis/node/core/BUILD.bazel
+++ b/metropolis/node/core/BUILD.bazel
@@ -39,5 +39,5 @@
     name = "core",
     embed = [":go_default_library"],
     pure = "on",  # keep
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis/node:__pkg__"],
 )