command-line-arguments : handling "undefined" error message

Following is how the source files are organized

✘-1 ~/Go/src/github.com/krmahadevan/packages 
18:24 $ tree .
.
├── sample_main.go
└── sample_one.go

0 directories, 2 files

Here's how the source code looks like:

sample_one.go

package main

var data map[string]string

func init() {
    data = make(map[string]string, 0)
}

sample_main.go

package main

import "fmt"

func main() {
    data["foo"] = "bar"
    fmt.Println(data)
}

Now when I attempt at running sample_main.go I get errors stating that data is undefined.

18:24 $ go run sample_main.go 
# command-line-arguments
./sample_main.go:6:2: undefined: data
./sample_main.go:7:14: undefined: data
✘-2 ~/Go/src/github.com/krmahadevan/packages

But when I build the code into a binary and then execute it, it runs fine.

✔ ~/Go/src/github.com/krmahadevan/packages 
18:27 $ go build
✔ ~/Go/src/github.com/krmahadevan/packages 
18:28 $ ./packages 
map[foo:bar]
✔ ~/Go/src/github.com/krmahadevan/packages

I would like to understand why is this behavior ?

Environment:

18:31 $ go version
go version go1.11.4 darwin/amd64

The closest I found was this post : Golang : command-line-arguments undefined: variable

But this post talks about scoped variables that are defined in main.

But my problem statement involves variables defined in another go file and accessed in the main method.