Go プロジェクトをビルドするときに「パッケージ XXX は GOROOT にありません」というメッセージが表示されるのはなぜですか?

package main

func addition(x int, y int) int {
    return x + y
}

package main

import "testing"

func TestAdd(t *testing.T) {

    t.Run("adding two positive numbers", func(t *testing.T) {
        sum := addition(2, 2)
        expected := 4

        if sum != expected {
            t.Errorf("Expected %d; but got %d", expected, sum)
        }
    })

    t.Run("adding two negative numbers", func(t *testing.T) {
        sum := addition(-3, -4)
        expected := -7

        if sum != expected {
            t.Errorf("Expected %d; but got %d", expected, sum)
        }
    })

    t.Run("adding one positive and one negative integer", func(t *testing.T) {
        sum := addition(1, -3)
        expected := -2

        if sum != expected {
            t.Errorf("Expected %d; but got %d", expected, sum)
        }
    })

}

package main

import "fmt"

func main() {
    var num1 int = 1
    var num2 int = 2

    sum := addition(num1, num2)
    product := multiplication(num1, num2)

    fmt.Printf("The sum of %d and %d is %d\n", num1, num2, sum)
    fmt.Printf("The multiplication of %d and %d is %d\n", num1, num2, product)
}