fbcode_builder: getdeps: add clean command
fbcode_builder: getdeps: add clean command

Summary:
This cleans up the scratch dir, removing everything except
for downloaded tarballs.

Reviewed By: simpkins

Differential Revision: D14781328

fbshipit-source-id: 9304e44a268cf7996c5e572a2eca219aefbf4b46

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly