Move to go modules based build

[chenrui333]

👋 In go 1.16, go modules would be the default build mode. And also godep project is archived, so it is time to move to go mod. Thanks!

[zees-dev]

Is there a roadmap on when this shall be possible?

[sam-thibault]

Related PR

• switch to go module #4116

[rfay]

This is super important, so you can get contributions from others. It's very difficult to explore this codebase or provide comments, bugfixes, anything else. Please do it, thanks!