registry: remove unused Units from FakeRegistry
job: unexport Requirements

There's no need for anything outside the package to access Requirements
directly; they should only be using the associated wrappers (e.g.
Peers(), Conflicts(), etc).
Merge pull request #832 from jonboulle/job_requirements

job: unexport Requirements
Merge pull request #831 from jonboulle/reg

registry: remove unused Units from FakeRegistry
api: add first pass at validation for units