MSBuildWorkspace NuGet package adds unexpected items in Solution Explorer

MSBuildWorkspace's NuGet package adds the build hosts via the content file mechanism; this can be surprising because those appear in Solution Explorer by default, which means people see a lot of cruft where the whole point is to be invisible. We may just want to inject a small bit of MSBuild to mark those as invisible.

(Ironically, I was involved in the project system/MSBuild implementation of content files ages ago, so the fact I forgot about such a behavior is funny...almost.)