Build our own packages using FrameworkReference

[natemcmaster]

Depends on NuGet/Home#7342.
Follow up to #4178

The packages we ship which reference the shared framework should be built using FrameworkReference. Once NuGet/Home#7342 is resolved, the metadata for this item will be preserved in the nuspec, which should ensure consumers get all the necessary runtime settings and compiler references.

Workaround
Consumers using our AspNetCore packages, such as Microsoft.AspNetCore.NodeServices, need to make sure they also have <FrameworkReference Include="Microsoft.AspNetCore.App" />. This will be implicitly added with Microsoft.NET.Sdk.Web, but not other MSBuild SDKs.

Changes required

1. We need to change our own packages to build with FrameworkReference. (Related work Convert projects in this repo to use ProjectReference #4246)
2. Update our SDK in BuildTools to support using FrameworkReference

[natemcmaster]

Prototype in progress: https://github.com/aspnet/AspNetCore/compare/natemcmaster:local-fx

Current challenges:

• I get compiler errors due to duplicate APIs in the same namespace. I think this is the result of having InternalsVisibleTo and duplicate copies of API due to shared source.
• Visual Studio go-to-definition is broken when navigating from something outside the shared framework into a shared framework assembly.
• Working around https://github.com/dotnet/core-setup/issues/4809 - the only way to make tests run against the shared framework is to install the shared framework into DOTNET_ROOT. My workaround is to make DOTNET_ROOT = "$repoRoot/.dotnet/$(arch)/".
• Requires VS 2019 and a Preview 2 SDK (blocked on Make Visual Studio 2019 a prerequisite to building this repo #7005).

cc @pakrym @ryanbrandenburg