Framework specific dependencies don't consider indirect dependencies

[terrajobst]

Ported from CodePlex

Repro:

1. NuProj references two projects: PortableLib and DesktopLib
2. PortableLib is portable, DesktopLib is .NET 4.5.1
3. DesktopLib depends on PortableLib
4. PortableLib references a NuGet package, DesktopLib doesn’t

Actual Behavior:

NuGet dependency is only added to portable, but not for .NET 4.5.1

Expected Behavior:

DesktopLib also depends on PortableLib, which is why it should also be referenced from portable