Dependency resolution has been reimplemented using a recursive algorithm

The new algorithm minimizes the search space while descending the dependency
graph. Compared to the old approach, it is now much less likely to run into
pathological cases that result in exponential run time ("The dependency
resolution algorithm is taking too long").

Furthermore, the error message in case of unsatisfiable dependencies is more
precise, usually making it straight forward to debug issues in the dependency
graph of a failing package.