API namespace cleanup for tag helpers

[Eilon]

From a meeting with @dougbu @rynowak @DamianEdwards

Move what’s in the Microsoft.AspNet.Razor.TagHelpers namespace to something like
Microsoft.AspNet.Razor.Compilation.TagHelpers.

And move what's in the Microsoft.AspNet.Razor.Runtime.TagHelpers namespace to Microsoft.AspNet.Razor.TagHelpers. An alternative that we considered was to rename it to Microsoft.AspNet.Html.TagHelpers, because it's not really Razor-specific, but meh.