Rewrite TARGETS files to BUCK files for facebook/fboss (#989)

Summary:
Pull Request resolved: #989

X-link: facebook/fboss#298

This diff migrates TARGETS file to BUCK files that are synced for an open source project.

Reviewed By: dtolnay

Differential Revision: D66561352

fbshipit-source-id: aaf9d08e996568659ecd2536fbba73311e63a3b1

Author: bigfootjon