Convert to go modules, update dependencies, fix integration tests and linter errors

[teutat3s]

Supersedes #594

This PR is a direct response to the recent comment by @danmcd in #594.

This is my first attempt at trying to split that big PR into multiple smaller ones, hopefully getting them merged.

Feedback welcome.

Testing notes:

• ran the following command successfully:

make clean build lint test integration

EDIT: Once this gets merged, I'll open follow-up PRs that work on top of this.

[travispaul]

Hi @teutat3s, thanks for your work on this. I had a couple of small comments but nothing crazy.