Replace //:protoc and similar default macro arguments with
@com_google_protobuf prefixed versions. This allows them to work in 3rd party
repositories.

Fix a bad visibility rule. :hidden does not exist, but :private does.
diff --git a/BUILD b/BUILD
index ee81c75..069b28e 100644
--- a/BUILD
+++ b/BUILD
@@ -254,7 +254,7 @@
     srcs = WELL_KNOWN_PROTOS,
     dest = "",
     strip_prefix = "src",
-    visibility = ["//visibility:hidden"],
+    visibility = ["//visibility:private"],
 )
 
 [proto_library(