Migrate to go.mod

[cbrewster]

the future is now, also this will let us more easily make a nix derivation

[replbot]

This PR is getting big. To make it easier for others to review you might want to breaking it up into smaller changes.

[replbot]

warning: This PR touches the dependencies!

When we're pulling in a dependency we're now maintaining it, it is no different from code we have in our repos.

Please make sure:

• [ ] You pull in new dependencies in a separate PR.
• [ ] You read and understand the code you're pulling in.
• [ ] Doesn't have security holes.
• [ ] It's doesn't add a lot to the bundle size.
• [ ] It isn't a solution that is over engineered.
• [ ] You consider vendoring it.

If you are upgrading a dependency please make sure to update related dependencies

If you're reviewing this PR, please review the dependencies and go checkout the source.

[replbot]

unbooping: Your PR is too powerful! Try breaking it up into multiple changes. If this is a pure refactor you can put [refactor] in the title.