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/kubernetes/hyperkube/BUILD b/metropolis/node/kubernetes/hyperkube/BUILD
index dced1c7..4c8c5c9 100644
--- a/metropolis/node/kubernetes/hyperkube/BUILD
+++ b/metropolis/node/kubernetes/hyperkube/BUILD
@@ -24,6 +24,6 @@
     name = "hyperkube",
     embed = [":go_default_library"],
     pure = "on",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis/node:__pkg__"],
     x_defs = version_x_defs(),
 )