Move from dep to go modules.
Review Request #11168 — Created Sept. 11, 2020 and submitted
Go finally has a built-in dependency management system and people have
moved away from the old solutions, including the semi-official
This change moves us over to the new modules system.
This also moves us to a github mirror of gohg, since the original
bitbucket repository is gone. We should move away from gohg at some
point because it's obviously completely unmaintained, but for the moment
it's still working.
- Did a test build.
- Ran unit tests.