How to persist "package" states in Go?

My goal is to encapsulate in one module/package.

Main package:

package main

import (
  "github.com/zenazn/goji"
  "./routes"
)

func main(){
  routes.Setup()
  goji.Serve()
}

And another package:

package routes

import "github.com/zenazn/goji"

func Setup() {
	goji.Get("/static", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprint(w, "static!")
	})
}

How can I do this?

goji, in your example, is a package. Not a variable.

You cannot pass packages around like this.
If you look at the example on the goji github page

You simply just call goji.Get from your Init function, and goji.Serve from your main

route.go

package route
import "route"
import "github.com/zenazn/goji"
func Init(){
    goji.Get("/hello/:name", hello)
}

main.go

package main
import "github.com/zenazn/goji"
func main(){
    route.Init()
    goji.Serve()
}

Packages in go export constants, variables, types and functions that have uppercase letters as their name. The package itself is not something directly manipulatable by the program.

The package goji should be exporting a variable named something like goji.Goji if you want to directly access it from other packages. A better solution is to provide some functions in the package that allow you to register your functions/helpers.

You could also export a function from goji like:

func Set(s string, func(w http.ResponseWriter, r *http.Request)) { ... }

that could be used by other packages:

goji.Set("/static", myFunc)

The error you had "use of package goji without selector" is saying you can't use the name of the package without specifying which exported value you want from the package. It's expecting goji.something not goji by itself.

The function init() inside go files has special properties: see http://golang.org/ref/spec#Program_initialization_and_execution