unexported-return: unexported interfaces should not be allowed

[tie]

Currently, unexported-return rule allows returning unexported interface types. E.g.

type unexportedInterface interface {
        unexportedMethod()
}

func New() unexportedInterface {
        return nil
}

As per discussion in #1456 (comment), this is unfriendly to the user of the package. The example above is also “annoying to use“ because the interface type includes an unexported method, so it cannot be assigned to a value that is a subset of the original interface in different package. E.g. https://go.dev/play/p/ohypfiXhADR

Expected behavior

The unexported-return rule should not allow named unexported return types (except when defined as an exported type alias).