fbcode_builder: getdeps: add `sandcastle` subcommand
fbcode_builder: getdeps: add `sandcastle` subcommand

Summary:
This command schedules a facebook specific sandcastle job for the current
commit in your repo for each of the platforms we have support for in
sandcastle.

You can use `--dry-run` to have it print out the job specs.

To support this, I've moved around some of the support utilities
to make it easier to import them.

Reviewed By: simpkins

Differential Revision: D14710330

fbshipit-source-id: fb1e2a2ce78e52894291159514977da97028b37f

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro