fix public types causing xmldoc warnings by making them internal

DepotDownloader/ContentDownloader.cs

@@ -11,7 +11,7 @@ using SteamKit2.CDN;
 
 namespace DepotDownloader
 {
-    public class ContentDownloaderException : Exception
+    class ContentDownloaderException : Exception
     {
         public ContentDownloaderException(String value) : base(value) { }
     }

DepotDownloader/PlatformUtilities.cs

@@ -3,7 +3,7 @@ using System.Runtime.InteropServices;
 
 namespace DepotDownloader
 {
-    public static class PlatformUtilities
+    static class PlatformUtilities
     {
         private const int ModeExecuteOwner = 0x0040;
         private const int ModeExecuteGroup = 0x0008;