It seems there is no change, as Jehof says.
ConfigurationBuilder is in its own package, as Jeroen Mostert says.
But make sure you also have the Microsoft.Extensions.Configuration.Json package, where the .AddJsonFile() extension lives.
In summary, you need the following two NuGet packages:
- Microsoft.Extensions.Configuration (2.0.0)
- Microsoft.Extensions.Configuration.Json (2.0.0)