Move RazorViewEngineOptionsSetup to an internal namespace

[rynowak]

RazorViewEngineOptionsSetup in the Microsoft.AspNetCore.Mvc namespace, and should be hidden in some gross internal place.

This is already marked as [Obsolete(...)] but I guess we never tracked moving it once we were open for breaking changes.

[Eilon]

Probably not announcement-worthy, given it's so obscure.