metropolis: unify utility packages

One last sweeping rename / reshuffle.

We get rid of //metropolis/node/common and //golibs, unifying them into
a single //metropolis/pkg meta-package.

This is to be documented somwhere properly, but here's the new logic
behind selecting where to place a new library package:

 - if it's specific to k8s-on-metropolis, put it in
   //metropolis/node/kubernetes/*. This is a self-contained tree that
   other paths cannot import from.
 - if it's a big new subsystem of the metropolis core, put it in
   //metropolis/node/core. This can be imported by anything in
   //m/n (eg the Kubernetes code at //m/n/kubernetes
 - otherwise, treat it as generic library that's part of the metropolis
   project, and put it in //metropolis/pkg. This can be imported by
   anything within //metropolis.

This will be followed up by a diff that updates visibility rules.

Test Plan: Pure refactor, CI only.

X-Origin-Diff: phab/D683
GitOrigin-RevId: 883e7f09a7d22d64e966d07bbe839454ed081c79
diff --git a/metropolis/pkg/supervisor/BUILD.bazel b/metropolis/pkg/supervisor/BUILD.bazel
new file mode 100644
index 0000000..40b0469
--- /dev/null
+++ b/metropolis/pkg/supervisor/BUILD.bazel
@@ -0,0 +1,28 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "go_default_library",
+    srcs = [
+        "supervisor.go",
+        "supervisor_node.go",
+        "supervisor_processor.go",
+        "supervisor_support.go",
+        "supervisor_testhelpers.go",
+    ],
+    importpath = "git.monogon.dev/source/nexantic.git/metropolis/pkg/supervisor",
+    visibility = [
+        "//metropolis/node:__subpackages__",
+        "//metropolis/test:__subpackages__",
+    ],
+    deps = [
+        "//metropolis/pkg/logtree:go_default_library",
+        "@com_github_cenkalti_backoff_v4//:go_default_library",
+        "@org_golang_google_grpc//:go_default_library",
+    ],
+)
+
+go_test(
+    name = "go_default_test",
+    srcs = ["supervisor_test.go"],
+    embed = [":go_default_library"],
+)