Go finally has a built-in dependency management system and people have

moved away from the old solutions, including the semi-official dep.

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.