v2+ Go modules

[applejag]

Based on RFC: iver-wharf/rfcs#26
Published: https://iver-wharf.github.io/rfcs/published/0026-v2-go-modules-release

Need to update the module names in the following repos to include the major version as suffix:

• wharf-api: Appended /v5 to Go module wharf-api#128
• wharf-api-client-go: Appended /v2 to Go module wharf-api-client-go#30

Motivation

We need to stay compatible with Go modules to be able to use this library from other projects. Go modules almost requires it's own doctrine, but this PR is based on some statements from here: https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher

Example errors that appear as we haven't been Go modules compatibility up until now:

$ go get github.com/iver-wharf/wharf-api/pkg/orderby@v4.2.0
go get github.com/iver-wharf/wharf-api/pkg/orderby@v4.2.0:
  github.com/iver-wharf/wharf-api@v4.2.0:
    invalid version:
      module contains a go.mod file, so major version must be compatible:
        should be v0 or v1, not v4

(I've wrapped the text in console output just to be more easily readable)