Make only root test_deps externally visible (#508)

All other test_deps targets implicitly require the root BUILD file
for the `license` target for their default_applicable_licenses;
therefore, users should only depend on the root test_deps target.

lib/BUILD

@@ -99,6 +99,7 @@ filegroup(
     name = "test_deps",
     testonly = True,
     srcs = ["BUILD"] + glob(["*.bzl"]),
+    visibility = ["//:__subpackages__"],  # Needs skylib's root BUILD file for default_applicable_licenses
 )
 
 # The files needed for distribution

rules/BUILD

@@ -69,6 +69,7 @@ filegroup(
     srcs = [
         "BUILD",
     ] + glob(["*.bzl"]),
+    visibility = ["//:__subpackages__"],  # Needs skylib's root BUILD file for default_applicable_licenses
 )
 
 # The files needed for distribution

toolchains/unittest/BUILD

@@ -56,7 +56,7 @@ filegroup(
     srcs = [
         "BUILD",
     ],
-    visibility = ["//:__subpackages__"],
+    visibility = ["//:__subpackages__"],  # Needs skylib's root BUILD file for default_applicable_licenses
 )
 
 # The files needed for distribution