Domain gopkg.in has a different V2 convention¶
Typically, a v2 go module path looks like github.com/xieyuschen/yaml/v3, however the gopkg.in has its own convention which is gopkg.in/yaml.v3.
Since gopkg.in shares the same version suffix methodology as Go modules, the Go command accepts the .v2 in gopkg.in/yaml.v2 as a valid major version suffix. This is a special case for compatibility with gopkg.in: modules hosted at other domains need a slash suffix like /v2.
So during manipulation the go import path by adding another domain, the addition changes the convention from gopkg.in to the standard practice. As a result, the go command reports an error given the following go.mod file.