fix netcoreapp3.0 aspnet reference and packaging

[baronfel]

As @tihan would say, this is a chungus of a change.

Did a few things:

• move to .net sdk tools for paket/fake
• update paket dependencies to pivot off of framework, since for the different frameworks we now have widely-diverging dependency groups
• conditional referencing of packages in each project's paket.references
• conditional reference to the framework aspnetcore app

after all of that, paket pack was broken because it doesn't understand the multi-group resolution, so instead we defer to dotnet pack. paket integrates well with this and passes along the versions for the different groups we want (and nuget gets the correct framework reference for aspnet when targeting netcoreapp3.0), but we lost the info in the paket.templates. So I added Directory.Build.props to set those properties.

The only code change required was some IFDEF around json parsing, because the types changed in aspnetcore 3.x

[isaacabraham]

Wonderful. I tried doing this a couple of months ago but gave up. Thank you for doing this - it will have a really positive effect not only in terms of keeping things up to date but also for "first load" times when creating Paket dependencies.