Moved PG extension builder to right place

src/EFCore.PG/Extensions/NpgsqlModelBuilderExtensions.cs

 using System;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
 using Microsoft.EntityFrameworkCore.Utilities;
 
 // ReSharper disable once CheckNamespace

src/EFCore.PG/NpgsqlPostgresExtensionBuilder.cs → src/EFCore.PG/Metadata/Builders/NpgsqlPostgresExtensionBuilder.cs

@@ -6,7 +6,7 @@
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore.Utilities;
 
-namespace Microsoft.EntityFrameworkCore.Metadata
+namespace Microsoft.EntityFrameworkCore.Metadata.Builders
 {
     public class NpgsqlPostgresExtensionBuilder
     {