Track dependencies more accurately in SwiftPMBuildSystem.targets(dependingOn:)

[ahoppen]

We currently use the topological sort to determine which targets depend on others. This is an over approximation because even targets that occur later in the topological sorting might be independent of the passed in target. We should be consulting the actual dependency graph instead of a flattened list here.

[ahoppen]

Synced to Apple’s issue tracker as rdar://128245755