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/logtree/BUILD.bazel b/metropolis/pkg/logtree/BUILD.bazel
index bb07e99..f13b39a 100644
--- a/metropolis/pkg/logtree/BUILD.bazel
+++ b/metropolis/pkg/logtree/BUILD.bazel
@@ -15,7 +15,7 @@
         "logtree_publisher.go",
     ],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/logtree",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
     deps = [
         "//metropolis/pkg/logbuffer:go_default_library",
         "//metropolis/proto/api:go_default_library",