* issue 4492: added xfailing test, added owner to DependencyMap
* DependencyMap.concrete checks if we have unconditional dependencies
This fixes#4492 and #4107 using some heuristics to avoid an infinite
recursion among Spec.satisfies, Spec.concrete and DependencyMap.concrete
The idea, as suggested by @becker33, is to check just for unconditional
dependencies. This is not covering the whole space and a package with
just conditional dependencies can still fail in the same way. It should
cover though all the **real** packages we have in our repo so far.