gomod

package main

import "mycore"

func main(){
    mycore.ShowName()
}

main.go

module mytest

require mycore v0.0.0

replace mycore v0.0.0 => ./mycore

gomod/go.mod

package mycore

import "fmt"

func ShowName(){
    fmt.Println("sunlong")
}

gomod/mycore/functions.go

module mycore

go 1.12

gomod/mycore/go.mod