Allow WorkingCopy resources to have multiple working copies

Summary:
Ref T9252. For building Phabricator itself, we need to have libphutil/, arcanist/ and phabricator/ next to one another on disk.

Expand the Drydock WorkingCopy resource so that it can have multiple repositories if the caller needs them.

I'm not sure if I'm going to put the actual config for this in Harbormaster or Drydock yet, but the WorkingCopy resource itself should work the same way in either case.

Test Plan: Restarted a Harbormaster build which leases a working copy, saw it build as expected.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14180