Allow Harbormaster to lease working copies from Drydock

Summary: Ref T9252. This is still crude in a few ways but basically works, at least for commits.

Test Plan:

• Made a build plan with just this build step.
• Ran bin/harbormaster build --plan 10 ... on a commit.
• It actually built a working copy, leased it, took no action, and released the lease. MAGIC~~~

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14160