Query classes should not have to be public

[darraghjones]

When I modify the sample code in the readme to remove the public qualifier on the GetFoo query, I get the following error:

Unhandled Exception: System.AggregateException: One or more errors occurred. ('object' does not contain a definition for 'ExecuteAsync') ---> Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'object' does not contain a definition for 'ExecuteAsync'
   at CallSite.Target(Closure , CallSite , Object , Object , CancellationToken )
   at System.Dynamic.UpdateDelegates.UpdateAndExecute3[T0,T1,T2,TRet](CallSite site, T0 arg0, T1 arg1, T2 arg2)
   at Paramore.Darker.PipelineBuilder`1.<BuildAsync>b__10_0(IQuery`1 r, CancellationToken ct)
   at Paramore.Darker.QueryProcessor.<ExecuteAsync>d__8`1.MoveNext()

However, in brighter, there is no requirement that commands are public.