mkdir gomodtest
cd gomodtest
go mod init gomodtest

package main
import "github.com/astaxie/beego"
func main() {
    beego.Run()
}

go run server.go
自动查找依赖并且下载依赖的包到$GOPATH/pkg/mod/github.com/astaxie
go run server.go
go: finding github.com/astaxie/beego v1.12.0
go: downloading github.com/astaxie/beego v1.
go: extracting github.com/astaxie/beego v1.
go: downloading golang.org/x/crypto v0.0.0-20181127143415-eb0de9b17e85
go: downloading gopkg.in/yaml.v2 v2.2.1
go: finding github.com/shiena/ansicolor latest
go: extracting gopkg.
go: downloading github.com/shiena/ansicolor v0.20151119151921-a422bbe96644
go: extracting golang.org/x/crypto v0.eb0de9b17e85
go: extracting github.com/shiena/ansicolor v0.a422bbe96644
go: finding gopkg.
go: finding golang.org/x/crypto v0.20181127143415-eb0de9b17e85

module gomodtest

go 1.13

require (
    github.com/astaxie/beego v1.12.0 // indirect
    github.com/shiena/ansicolor v0.20151119151921-a422bbe96644  indirect
)