Support multi-targeting with NoTargets and/or Traversal SDKs

[kzu]

Both NoTargets and Traversal SDKs are very convenient for creating packaging projects.

NoTargets, as a simple aggregator of content for a single-targeting (one TF) package. Traversal for multi-package building from a single entry point.

Once scenario that isn't working right now, is for multi-targeting packaging projects. Neither SDK will actually pack the "outer" packaging project itself.

We could also just introduce a NuGetizer SDK too, based off of the Traversal SDK most likely.