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/fsquota/fsxattrs/BUILD.bazel b/metropolis/pkg/fsquota/fsxattrs/BUILD.bazel
index 87f2617..87fc9e1 100644
--- a/metropolis/pkg/fsquota/fsxattrs/BUILD.bazel
+++ b/metropolis/pkg/fsquota/fsxattrs/BUILD.bazel
@@ -4,6 +4,6 @@
     name = "go_default_library",
     srcs = ["fsxattrs.go"],
     importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/fsquota/fsxattrs",
-    visibility = ["//visibility:public"],
+    visibility = ["//metropolis:__subpackages__"],
     deps = ["@org_golang_x_sys//unix:go_default_library"],
 )