Motivated by https://github.com/NuGet/Home/wiki/Global-Tools---NuGet-Implementation/

@rrelyea thinks that we should have already been enforcing 1 package type per package, but currently that's allowed by both specification and the implementation of Pack.

Update:

Pack doesn't validate.
Install through PM UI validates.
Adding in the package reference manually and restoring doesn't validate.

This would be a breaking change.