`use`包范围:如何让它跨文件？

// file1.go
package main

import "fmt"

var MyPackage = fmt

// file2.go
package main

func main() {
    MyPackage.Println("Hello, world!")
}

// file1.go
package main

import "fmt"

type MyInterface interface {
    Println(a ...interface{}) (n int, err error)
}

// file2.go
package main

import "fmt"

type MyPackage struct{}

func (m MyPackage) Println(a ...interface{}) (n int, err error) {
    return fmt.Println(a...)
}

// file3.go
package main

func main() {
    var myPackage MyInterface = MyPackage{}
    myPackage.Println("Hello, world!")
}