Remove all uses of gopkg.in

[ptzianos]

Issue

Golang has had multiple iterations of official ways of handling dependencies. They all failed in many, unique and often entertaining ways. Finally, Go decided to use the same method that lot's of other languages use (Python, JS, Rust ...), using a go.mod finally to list the packages and a go.sum to mark the checksums of the packages are pulled. gopkg.in was a proxy used before the advent of proper Go tooling for dependency management and its no longer needed.

Expected behaviour

Use just go.mod for dependencies

Actual behaviour

There are still leftover gopkg.in references in the code

[linear]

OCU-30 Remove all uses of `gopkg.in`

[ptzianos]

Fixed as part of #53