Allow creation of ReachMap without "poisoning"

[sdboyer]

The current algorithm for extracting a ReachMap from a PackageTree is a depth-first traversal that not only eliminates packages with errs from from the PackageTree, but also uses backpropagation to eliminate packages that transitively import those packages. Internally, that backpropagation method is referred to as "poisoning."

This approach is always what the solver needs, but it's not necessarily what gps' dependers need. Implementations are often scanning the current project's code, and want to be more lenient about what gets eliminated. So, we need more params, or another method, that takes that approach to creating a ReachMap.