I'm not sure that we are on the same page

Are you saying that you want multiple build tagged files each with a different implementation of the function, all in the same package? (eg. windows, linux, arm)

I mean, the example given by the GP is two implementations in the same package, the standard library version is used in the prod file and the test implementation in the test files - the _test.go is the (implicit) build tag

Or do you have something else in mind?