Issue Description:

It is convenient to separate out default values of a model into a separate package, so they can be used in multiple stacks for testing (E.g. CoreData tests, Fluent tests on the server etc…)

So given a `Model` package, I would like to use a `TestSupport` package that defines default values for various mocks. For that, the `TestSupport` package needs to import `Model` to extend its types.

Now the `Model` package's tests want to import the `TestSupport` package to make use of the mocks in there, but this crashes Xcode and SPM (but correctly reports as cyclic dependency when using the pubgrub resolver)

If SPM supported cyclic dependencies, this would allow for very clean separation of packages .