Ensure NuSpecs for transport packages conform to NuGet guidance

While creating https://github.com/dotnet/wpf/pull/2138, we ran into NU5128 and NU5131.

According to NuGet guidance, we should be creating a reference group for all of our reference DLLs ([see here](https://docs.microsoft.com/en-gb/nuget/create-packages/select-assemblies-referenced-by-projects)).

This should fix both warnings.

NU5128
```
Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location.
```

NU5131
```
References were found in the nuspec, but some reference assemblies were not found in both the nuspec and ref folder.
```