Enable auto-generating sources rules

pkg/util/async/BUILD

@@ -20,3 +20,16 @@ go_test(
     library = ":go_default_library",
     tags = ["automanaged"],
 )
+
+filegroup(
+    name = "package-srcs",
+    srcs = glob(["**"]),
+    tags = ["automanaged"],
+    visibility = ["//visibility:private"],
+)
+
+filegroup(
+    name = "all-srcs",
+    srcs = [":package-srcs"],
+    tags = ["automanaged"],
+)