Fixes #113 and potentially #245 which were broken by recent changes

PrePrepareFile should be an override method but the change create a new
method hiding the base version.

uSync.Migrations.Core/Handlers/Shared/SharedDataTypeHandler.cs

@@ -92,8 +92,8 @@ protected override void PrepareFile(XElement source, SyncMigrationContext contex
             context.DataTypes.GetByDefinition(dtd)!.OriginalEditorAlias = editorAlias;
         }
     }
-
-    public void PrePrepareFiles(XElement source, SyncMigrationContext context)
+    
+    protected override void PrePrepareFile(XElement source, SyncMigrationContext context)
     {
         var editorAlias = GetEditorAlias(source);
         var (alias, dtd) = GetAliasAndKey(source, context);