Transitive dependencies of nuget packages don't contribute to build

Hi,

I'm running into a problem with using the `xunit` package from nuget with the paket/bazel integration.
The xunit package is itself empty, as far as I'm aware, and transitively depends on `xunit.core`, which in turn depends on `xunit.extensibility.core` which has the actual DLL which dotnet needs to compile against.

I have setup a simple project at https://github.com/criemen/rules-dotnet-repro, which compiles with `dotnet build`, but fails to compile with `bazelisk build //:Program`, as bazel can't find the `Xunit` namespace.
Looking into `Program.dll-0.params`, this seems to be the case as the transitive dependency `xunit.extensibility.core` doesn't contribute its DLL to the build invocation.

Adding `@paket.main//xunit.extensibility.core` to the deps of the project fixes the build, as expected.
As this is an implementation detail of the `xunit` package, as user I don't want to think about that and specify that dependency by hand.