job-manager: allow dependencies on completed jobs

[garlick]

Some other random thoughts:

• The dependency plugins in the job-manager could also make great use of the inactive job cache to allow jobs to have dependencies on other, completed jobs (right now jobs can only have dependencies on active jobs). This might only work if the job "result" for inactive jobs is available.
• Apropos of the previous bullet, it might be nice to have a function to lookup a jobid that checks active hash and falls back to inactive hash. Callers can check the job state to determine if the job was active vs inactive to determine from which hash the job was found.

Originally posted by @grondo in #4286 (comment)

[garlick]

This seems like a good and relatively easy improvement to dependencies, so adding to the next milestone.

It seems like rfc 26 does not place any restrictions on the antecedent job state when a simple job dependency is added, so this is actually required for a correct implementation?

[grondo]

Yes, but RFC 26 describes the dependency specification and I don't think covers details of how dependencies are satisfied? But I agree it would be more correct and less surprising to fix this particular issue. (Though if there is an inactive job purge, I'm guessing the same problem could be introduced, so I would avoid explicitly requiring that any inactive job can be a antecedent...)