unexported-return: do not allow unexported interfaces

[tie]

The current implementation of unexported-return rule allows returning unexported interface types.

type unexportedInterface any

func New() unexportedInterface {
        return nil
}

Closes #1457

[veeam-denis]

Note, having pointer for an interface type as a result in 99.99% of cases is a mistake. Although, we can check it, I'd focus on a non-pointer result.

[ccoVeille]

Should we have a rule for detecting this? Or is it something already detected?

I have no computer with me to check unfortunately

[chavacava]

Returning an interface (vs concrete type) is usually not the best option (unless the func is a struct factory or something like that)

[veeam-denis]

@chavacava I agree, but here we have a pointer to an interface, which is even worse :)

I can imagine returning a settable value of an interface type and for that we return a pointer to it, but that's something I personally never saw in real life (neither I had a reason to do that in my own code). I saw this for input arguments that were meant to be the output ones as well, but not for the returned value.