Provide mechanism to mark packages as PackageReference style only

[dasMulli]

Similar to "minimum client version", a nuspec should have a feature to indicate that the package was authored for consumption only for packages using PackageReference.

Currently, package authors have to add targets to create a build error when the package was installed using packages.config.

There should be a path forward for allowing maintainers to drop packages.config support, especially when the supported platforms will only use SDK-style projects (.NET Core, upcoming Xamarin, possibly UWP projects) or PackageReference by default.
This migration path should create an error on package install rather than build to be visible to the user and guide the user to PackageReference migration documentation.

[rrelyea]

@nkolev92 - can you noodle on this with your other PR related work this sprint? This would have a downside of not erroring in way old versions. But we should think this through.

[dasMulli]

This would have a downside of not erroring in way old versions.

I guess this what's minimum client version in the nuspec was for. So it would error on old versions and error for packages.config in new versions.

There are just scenarios that are way complicated to make work in both worlds. contentFiles comes to my mind first. Then ref/runtimes splits etc.

At some point it would be like "built with .NET Core in mind but should also work on UWP/Mono and even .NET if you care upgrading the projects to PackageReference".