C:\Users\wangy>D:
D:\>cd GoCodes
D:\GoCodes>mkdir go-utils
D:\GoCodes>cd go-utils
D:\GoCodes\go-utils>go mod init github.com/wangy8961/go-utils
go: creating new go.mod: module github.com/wangy8961/go-utils

D:\GoCodes\go-utils>mkdir greetings
D:\GoCodes\go-utils>cd greetings

package greetings

import "fmt"

// Hello returns a greeting for the named person.
func Hello(name string) string {
    // Return a greeting that embeds the name in a message.
    message := fmt.Sprintf("Hi, %v. Welcome!", name)
    return message
}

D:\GoCodes\go-utils\greetings>cd ..\..
D:\GoCodes>mkdir hello
D:\GoCodes>cd hello

package main

import (
    "fmt"

    "github.com/wangy8961/go-utils/greetings"
)

func main() {
    // Get a greeting message and print it.
    message := greetings.Hello("Madman")
    fmt.Println(message)
}

D:\GoCodes\hello>go run hello.go
hello.go:6:2: cannot find module providing package github.com/wangy8961/go-utils/greetings: working directory is not part of a module

D:\GoCodes\hello>go mod init hello
go: creating new go.mod: module hello

D:\GoCodes\hello>go run hello.go
go: finding module for package github.com/wangy8961/go-utils/greetings
hello.go:6:2: cannot find module providing package github.com/wangy8961/go-utils/greetings: module github.com/wangy8961/go-utils/greetings: git ls-remote -q origin in D:\GoCodes\pkg\mod\cache\vcs\4198013b20472f12383aa49f4d08ebf23a98c0d78ed27e0079374f5ac333bd32: exit status 128

module hello

go 1.15

replace github.com/wangy8961/go-utils => ../go-utils

D:\GoCodes\hello>go build
go: found github.com/wangy8961/go-utils/greetings in github.com/wangy8961/go-utils v0.0.0-00010101000000-000000000000

module hello

go 1.15

replace github.com/wangy8961/go-utils => ../go-utils

require github.com/wangy8961/go-utils v0.0.0-00010101000000-000000000000

D:\GoCodes\hello>hello.exe
Hi, Madman. Welcome!