Internalization of OneToOne and ManyToOne Column classes (#2632)

* Internalization of OneToOne and ManyToOne Column classes

* PR feedback.

Author: codemzs

src/Microsoft.ML.Data/Transforms/ColumnBindingsBase.cs

@@ -14,7 +14,7 @@
 
 namespace Microsoft.ML.Data
 {
-    public abstract class SourceNameColumnBase
+    internal abstract class SourceNameColumnBase
     {
         [Argument(ArgumentType.AtMostOnce, HelpText = "Name of the new column", ShortName = "name")]
         public string Name;
@@ -103,13 +103,15 @@ public bool TrySanitize()
         }
     }
 
-    public abstract class OneToOneColumn : SourceNameColumnBase
+    [BestFriend]
+    internal abstract class OneToOneColumn : SourceNameColumnBase
     {
         [BestFriend]
         private protected OneToOneColumn() { }
     }
 
-    public abstract class ManyToOneColumn
+    [BestFriend]
+    internal abstract class ManyToOneColumn
     {
         [Argument(ArgumentType.AtMostOnce, HelpText = "Name of the new column", ShortName = "name")]
         public string Name;

src/Microsoft.ML.Transforms/Text/WordHashBagProducingTransform.cs

@@ -161,7 +161,7 @@ internal static IDataTransform Create(IHostEnvironment env, Options options, IDa
     /// </summary>
     internal static class NgramHashExtractingTransformer
     {
-        public abstract class ColumnBase : ManyToOneColumn
+        internal abstract class ColumnBase : ManyToOneColumn
         {
             [Argument(ArgumentType.AtMostOnce, HelpText = "Ngram length (stores all lengths up to the specified Ngram length)", ShortName = "ngram")]
             public int? NgramLength;