Issue Description:

Use-case: long-lived tools like an IDE plugin can use libSwiftPM to discover information about a package (package graph, build plan, etc.), but currently have no supported way to detect when that information is out of date and needs to be recomputed. They can watch the filesystem themselves, but would have to guess what changes are relevant.

libSwiftPM should provide this functionality in some form. A minimalist solution would be to expose enough information so that clients of SwiftPM can watch the right set of paths and determine when their information is (potentially) out-of-date and should be re-queried. A more advanced solution would be to do the work of detecting changes in SwiftPM and allow clients to just subscribe to change notifications.