Use shproj for shared code

Currently, there's just one task in the repo, as this grows, we will likely need to begin sharing code between packages. When that occurs, we should consider using an shproj to share code rather than manual linking.

There is some discussion around support for using shared projects.

> jaredpar 21 hours ago
Why do all the manual likning here vs. creating a shared source project?

> natemcmaster 21 hours ago
Is shproj compatible with dotnet-CLI now?

> chcosta 21 hours ago
At the moment, this isn't even needed, these could just be moved into the only build project in the repo. I'll do that for now.

> tmat 21 hours ago
Is shproj compatible with dotnet-CLI now?
Were there any issues? We use shared projects in Roslyn.

> natemcmaster 20 hours ago
Last I checked, $(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\CodeSharing\ doesn't exist in the msbuild that ships in .NET Core CLI

> tmat commented 20 hours ago
@natemcmaster Microsoft.CodeSharing.*.targets don't seem to do anything useful for the build. They are just setting project capabilities and such, presumably for design-time build in VS. Since they are imported conditionally it should work without them. @nguerrera @davkean Do shared projects work in VS Code/VS Mac?

> jaredpar 3 hours ago
Unsure about that particular MSBuild mechanism but we definitely use shared source projects in Roslyn + dotnet/CLI to build them.