use go mod to build

[guilhem]

• remove submodules
• Updates dependencies to reflect imports

[purpleidea]

Hi! Thanks for the patch. Unfortunately we don't want to move to go modules at this time, because we want to develop on git master. We'd like to be able to do this in a different way. More discussion here: #572

If you're blocked for some reason because of this please let me know why.

Thanks!