internal/pkg: make checksum available to cure

This enables deduplication by value as implemented in execArtifact.

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

cmd/mbf/main.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"syscall"
+	"unique"
 
 	"hakurei.app/command"
 	"hakurei.app/container"
@@ -113,7 +114,7 @@ func main() {
 			_, _, _, stage3 := rosa.Std.NewLLVM()
 			var (
 				pathname *check.Absolute
-				checksum [2]pkg.Checksum
+				checksum [2]unique.Handle[pkg.Checksum]
 			)
 
 			if pathname, checksum[0], err = cache.Cure(stage2); err != nil {
@@ -127,8 +128,8 @@ func main() {
 
 			if checksum[0] != checksum[1] {
 				err = &pkg.ChecksumMismatchError{
-					Got:  checksum[0],
-					Want: checksum[1],
+					Got:  checksum[0].Value(),
+					Want: checksum[1].Value(),
 				}
 			}
 			return