Figure out the story for P2P references + TFMs + Microsoft.AspNetCore.Razor.Design

[rynowak]

Using P2P reference to build this package is causing a lot of trouble due to dotnet/msbuild#2661 and some other design issues.

This package should logically target netstandard2.0, but has build dependencies on projects that target net46 and netcoreapp2.0. So far I've worked around these issues enough to make progress, but this isn't really in good shape. We should take a fresh look at these issues and come up with a good solution.

[rynowak]

I think we've arrived at the ideal solution here. I am not aware of any additional work we need to do.