Update TypeSourceSelector.cs

dotnet/SqlClient#1930

> I had the same issue here. After changing to GetExportedTypes() seems to fix the issue

src/Scrutor/TypeSourceSelector.cs

@@ -154,7 +154,7 @@ private IImplementationTypeSelector InternalFromAssembliesOf(IEnumerable<Type> t
 
     private IImplementationTypeSelector InternalFromAssemblies(IEnumerable<Assembly> assemblies)
     {
-        return AddSelector(assemblies.SelectMany(asm => asm.DefinedTypes).Select(x => x.AsType()));
+        return AddSelector(assemblies.SelectMany(asm => asm.ExportedTypes);
     }
 
     private static IEnumerable<Assembly> LoadAssemblies(IEnumerable<AssemblyName> assemblyNames)