message: relocate from container

This package is quite useful. This change allows it to be imported without importing container.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/executable_test.go

@@ -5,11 +5,12 @@ import (
 	"testing"
 
 	"hakurei.app/container"
+	"hakurei.app/message"
 )
 
 func TestExecutable(t *testing.T) {
 	for i := 0; i < 16; i++ {
-		if got := container.MustExecutable(container.NewMsg(nil)); got != os.Args[0] {
+		if got := container.MustExecutable(message.NewMsg(nil)); got != os.Args[0] {
 			t.Errorf("MustExecutable: %q, want %q",
 				got, os.Args[0])
 		}