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/proto/common/BUILD.bazel b/metropolis/proto/common/BUILD.bazel
index 882bb61..a5f7eb0 100644
--- a/metropolis/proto/common/BUILD.bazel
+++ b/metropolis/proto/common/BUILD.bazel
@@ -5,19 +5,19 @@
 proto_library(
     name = "common_proto",
     srcs = ["common.proto"],
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
 )
 
 go_proto_library(
     name = "common_go_proto",
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/proto/common",
     proto = ":common_proto",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
 )
 
 go_library(
     name = "go_default_library",
     embed = [":common_go_proto"],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/proto/common",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
 )