AppHost: Support for .NET Core targeting apps

[AArnott]

When AppHost is installed in a .NET Core targeting application, it fails to scan the $(TargetPath) assembly because it can't load the System.Runtime ref assembly (at least when the MSBuild task runs on .NET Framework).

I suspect the fix for this is to teach the MSBuild Task where to find the .NET Framework runtime version of System.Runtime and load that.

Also: add the right package dependencies to the AppHost package for the other frameworks.