Tell user about required ClangSharp.Pathogen support packages at compile time

[PathogenDavid]

If you're trying to use Biohazrd on a platform besides Windows x64 or Linux x64, you need to install an additional ClangSharp.Pathogen runtime support package for your platform. Biohazrd automatically tells you which package you're missing at runtime, but it'd be nice if we also used an MSBuild target to tell you at compile time. (We can't automatically add a PackageReference for you because packages which add package references cause really wonky restore behavior.)