--- File pkg1/a.go
type A struct {
    b B
}

func (a A) YouWanted() string{
    return a.b.ProcessString()
}

--- File pkg1/b.go

type B struct {
    s string
}

type (b B) ProcessString() string {
    return strings.TrimPrefix(b.s, "aaaaa")
}


--- File pkg2/jonsnow.go

func JonSnow(something A) string {
    myString := a.YouWanted()
    // JonSnow knows nothing and expect to have a particoular string from this

    //...
    return ""
}