[packages] Add warnings for targets known to infrastructure

This CL is part of the work to delete //topaz/packages. There are a
handful of targets in //topaz/packages that are known to infrastructure
and need to be preserved in order to not break infrastructure.

Change-Id: Ic2300c9dfa795abbc7b9281d0d30b69f850fb8a9
diff --git a/packages/sdk/BUILD.gn b/packages/sdk/BUILD.gn
index 80be420..e6776f8 100644
--- a/packages/sdk/BUILD.gn
+++ b/packages/sdk/BUILD.gn
@@ -9,6 +9,7 @@
   ]
 }
 
+# WARNING: Known to infrastructure.
 group("topaz") {
   testonly = true
   public_deps = [
diff --git a/packages/tests/BUILD.gn b/packages/tests/BUILD.gn
index 97bd4a7..c9039eb 100644
--- a/packages/tests/BUILD.gn
+++ b/packages/tests/BUILD.gn
@@ -57,6 +57,7 @@
   ]
 }
 
+# WARNING: Known to infrastructure.
 group("all") {
   testonly = true
   public_deps = [
diff --git a/packages/tools/BUILD.gn b/packages/tools/BUILD.gn
index 27ac4b0..f0cffea 100644
--- a/packages/tools/BUILD.gn
+++ b/packages/tools/BUILD.gn
@@ -24,6 +24,7 @@
   ]
 }
 
+# WARNING: Known to infrastructure.
 group("doc_checker") {
   testonly = true
   public_deps = [