Can't reference DLL from GAC

[bjorkstromm]

We currently assume all #r directives are file paths.

[devlead]

@bjorkstromm is this still relevant since we bumped to .NET 3.1/5/6?

[bjorkstromm]

@bjorkstromm is this still relevant since we bumped to .NET 3.1/5/6?

Dunno, need to verify if this is still relevant. There's no GAC anymore, and additional System.* libs should be pulled via NuGet.