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/pkg/freeport/BUILD.bazel b/metropolis/pkg/freeport/BUILD.bazel
index 8ac6daf..70084b2 100644
--- a/metropolis/pkg/freeport/BUILD.bazel
+++ b/metropolis/pkg/freeport/BUILD.bazel
@@ -4,5 +4,5 @@
     name = "go_default_library",
     srcs = ["freeport.go"],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/freeport",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
 )