NuGet: Fix handling of packages with AssemblyReferences but not PackageAssemblyReferences

The previous implementation was breaking with Math.NET Numerics:

* This package also supports Silverlight 5.0, whose version "5.0" is higher than the "4.5" of .Net 4.5. The old implementation did choose the Silverlight build.
* This package includes a build for .Net 4.0 but not a special build for .Net 4.5. The .Net 4.0 build also supports .Net 4.5. The old implementation did an exact match, missing the .Net 4.0 build and thus not referencing anything.