Make artifacts imply dependencies on build steps

Summary: This makes input artifacts imply the appropriate build step dependencies in the build plan.  That is, if you use a host artifact in a build step, it will then implicitly depend on the 'Lease Host' step.

Test Plan: Viewed the build plan with the artifacts, saw the dependencies.  Ran a build, saw everything execute in the correct order.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10089