R1:bfaa93aa9b2c

Allow Harbormaster build plans to request additional working copies

Summary:
Ref T9123. To run upstream builds in Harbormaster/Drydock, we need to be able to check out `libphutil`, `arcanist` and `phabricator` next to one another.

This adds an "Also Clone: ..." field to Harbormaster working copy build steps so I can type all three repos into it and get a proper clone with everything we need.

This is somewhat upstream-centric and a bit narrow, but I don't think it's totally unreasonable, and most of the…