Use netstandard for coreservices DLL (#676)

* Use netstandard for coreservices DLL
- This is possible thanks to recent updates that target SMO bits using netstandard

* Fix localization failure during dotnet pack
- Namespace changes on SRGen, so updated references to accept the name change

src/Microsoft.SqlTools.Hosting.v2/Extensibility/RegisteredServiceProvider.cs

@@ -9,6 +9,7 @@ using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
 using Microsoft.SqlTools.Hosting.Utility;
+using Microsoft.SqlTools.Hosting.v2;
 
 namespace Microsoft.SqlTools.Hosting.Extensibility
 {