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.