getdeps: fix a crash if the project_hashes directory does not exist
getdeps: fix a crash if the project_hashes directory does not exist

Summary:
This makes getdeps.py no longer crash if used in the folly repository.
Folly does not depend on any other Facebook projects, so it does not
include a `build/deps` directory.

Reviewed By: wez

Differential Revision: D16778009

fbshipit-source-id: 41be53d862f41b62154b081eb90ddba8742658fe

facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman