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/tpm/BUILD.bazel b/metropolis/pkg/tpm/BUILD.bazel
index d06ff37..7ea5b87 100644
--- a/metropolis/pkg/tpm/BUILD.bazel
+++ b/metropolis/pkg/tpm/BUILD.bazel
@@ -7,7 +7,7 @@
         "tpm.go",
     ],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/tpm",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
     deps = [
         "//metropolis/pkg/logtree:go_default_library",
         "//metropolis/pkg/sysfs:go_default_library",
diff --git a/metropolis/pkg/tpm/eventlog/BUILD.bazel b/metropolis/pkg/tpm/eventlog/BUILD.bazel
index 94a7ee9..78b3ed4 100644
--- a/metropolis/pkg/tpm/eventlog/BUILD.bazel
+++ b/metropolis/pkg/tpm/eventlog/BUILD.bazel
@@ -8,7 +8,7 @@
         "secureboot.go",
     ],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/tpm/eventlog",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
     deps = [
         "//metropolis/pkg/tpm/eventlog/internal:go_default_library",
         "@com_github_google_certificate_transparency_go//x509:go_default_library",