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/supervisor/BUILD.bazel b/metropolis/pkg/supervisor/BUILD.bazel
index 40b0469..ac00fe7 100644
--- a/metropolis/pkg/supervisor/BUILD.bazel
+++ b/metropolis/pkg/supervisor/BUILD.bazel
@@ -10,10 +10,7 @@
         "supervisor_testhelpers.go",
     ],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/supervisor",
-    visibility = [
-        "//metropolis/node:__subpackages__",
-        "//metropolis/test:__subpackages__",
-    ],
+    visibility = ["//metropolis:__subpackages__"],
     deps = [
         "//metropolis/pkg/logtree:go_default_library",
         "@com_github_cenkalti_backoff_v4//:go_default_library",