Summary:
Our current lack of dependency management is leading to issues with
Debian packaging and risks inconsistent builds. Let's use `dep`, the
"official experiment" for go dependency management, and commit the
vendor directory which is small enough following a `dep prune`.

Fixes T180530
Depends on D881

Test Plan:
Run `make` or `go build -v` and examine output to ensure use of `vendor/`
packages.

Reviewers: thcipriani, Joe, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Maniphest Tasks: T180530

Differential Revision: https://phabricator.wikimedia.org/D882