Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48

cc/Android.bp

@@ -116,4 +116,6 @@ bootstrap_go_package {
         "cmake_module_cc.txt",
     ],
     pluginFor: ["soong_build"],
+    // Used by plugins
+    visibility: ["//visibility:public"],
 }