Adopt Go modules

[na--]

After Go 1.13 comes out, likely sometime next month, we should probably replace dep with the official Go modules:

• https://github.com/golang/go/wiki/Modules
• https://blog.golang.org/modules2019
• https://blog.golang.org/using-go-modules
• https://blog.golang.org/migrating-to-go-modules

[wingyplus]

I would like to help this issue if nobody working on it. :D

[cuonglm]

@wingyplus Thanks.

If you want to work on it, consider to update CI/CD, too.

[na--]

For posterity, it turns out that there are some serious deficiencies regarding vendoring support with Go modules that I didn't expect at all... I've collected a summary here: #1150 (comment)

[na--]

Something else we should adopt when we switch to Go modules: https://golang.org/pkg/runtime/debug/#ReadBuildInfo

Basically, it would be very useful to have the exact commit ID in each k6 binary, as part of k6 version and/or as a debug console message.