Update core allowed dependencies

This list has gotten a little out-of-date. Note that `clockwork` used to be
used but was removed in gvisor.dev/pr/5384.

PiperOrigin-RevId: 390644841

CONTRIBUTING.md

@@ -69,6 +69,10 @@ Rules:
             binary). Use `//pkg/unet` instead.
     *   `@org_golang_x_sys//unix:go_default_library` (Go import
         `golang.org/x/sys/unix`).
+    *   `@org_golang_x_time//rate:go_default_library` (Go import
+        `golang.org/x/time/rate`).
+    *   `@com_github_google_btree//:go_default_library"` (Go import
+        `github.com/google/btree`).
     *   Generated Go protobuf packages.
     *   `@org_golang_google_protobuf//proto:go_default_library` (Go import
         `google.golang.org/protobuf`).

WORKSPACE

@@ -669,13 +669,6 @@ go_repository(
     version = "v0.0.0-20160803190731-bd40a432e4c7",
 )
 
-go_repository(
-    name = "com_github_jonboulle_clockwork",
-    importpath = "github.com/jonboulle/clockwork",
-    sum = "h1:VKV+ZcuP6l3yW9doeqz6ziZGgcynBVQO+obU0+0hcPo=",
-    version = "v0.1.0",
-)
-
 go_repository(
     name = "com_github_jtolds_gls",
     importpath = "github.com/jtolds/gls",