Internalization of common inputs.

Author: TomFinley

src/Microsoft.ML.Data/EntryPoints/InputBase.cs

@@ -221,7 +221,8 @@ public static TOut Train<TArg, TOut>(IHost host, TArg input,
     /// <summary>
     /// Common input interfaces for TLC components.
     /// </summary>
-    public static class CommonInputs
+    [BestFriend]
+    internal static class CommonInputs
     {
         /// <summary>
         /// Interface that all API transform input classes will implement.