fbcode_builder: getdeps: add copytree helper
fbcode_builder: getdeps: add copytree helper

Summary:
this module adds some functions that help with copying
directory trees.  The copytree function is aware of eden and will
issue a prefetch prior to walking the directory.

Reviewed By: simpkins

Differential Revision: D14691006

fbshipit-source-id: 079bf850756f61aca17978453d07bc73b2f91788

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