Use phutil functions to copy/move files

Summary:
The cp and mv commands, when run from a Windows
environment, error out. Use library functions from Filesystem
for cross-platform compatibility.

Test Plan:
Ran arc lint with auto-fix lint errors on both Linux and
Windows.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Spies: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D16273