Now that sourcelink is a native part of the .NET SDK, Arcade doesn't
bring sourcelink packages in anymore. Now that the cyclic package
dependency is avoided, these projects don't need to use nuspecs
anymore.

This removes custom infrastructure and allows better source build
controls.

I diffed the produced packages and the content in the .NETCoreApp
folder is identical (a deps.json file is added but that's
recommended by msbuild for build tasks these days). The .NET Framework
output is significantly different as it now includes all dependencies
that aren't supplied by the MSBuild inside Visual Studio.