internal/pkg: make checksum available to cure

This enables deduplication by value as implemented in execArtifact.

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

internal/pkg/pkg_test.go

@@ -159,7 +159,7 @@ func destroyArtifact(
 	} else {
 		p := base.Append(
 			"checksum",
-			pkg.Encode(checksum),
+			pkg.Encode(checksum.Value()),
 		)
 		if err = filepath.WalkDir(p.String(), func(
 			path string,
@@ -362,20 +362,34 @@ var ignorePathname = check.MustAbs("/\x00")
 func cureMany(t *testing.T, c *pkg.Cache, steps []cureStep) {
 	t.Helper()
 
+	makeChecksumH := func(checksum pkg.Checksum) unique.Handle[pkg.Checksum] {
+		if checksum == (pkg.Checksum{}) {
+			return unique.Handle[pkg.Checksum]{}
+		}
+		return unique.Make(checksum)
+	}
+
 	for _, step := range steps {
 		t.Log("cure step:", step.name)
 		if pathname, checksum, err := c.Cure(step.a); !reflect.DeepEqual(err, step.err) {
 			t.Fatalf("Cure: error = %v, want %v", err, step.err)
 		} else if step.pathname != ignorePathname && !pathname.Is(step.pathname) {
 			t.Fatalf("Cure: pathname = %q, want %q", pathname, step.pathname)
-		} else if checksum != step.checksum {
-			t.Fatalf("Cure: checksum = %s, want %s", pkg.Encode(checksum), pkg.Encode(step.checksum))
+		} else if checksum != makeChecksumH(step.checksum) {
+			t.Fatalf(
+				"Cure: checksum = %s, want %s",
+				pkg.Encode(checksum.Value()), pkg.Encode(step.checksum),
+			)
 		} else {
 			v := any(err)
 			if err == nil {
 				v = pathname
 			}
-			t.Log(pkg.Encode(checksum)+":", v)
+			var checksumVal pkg.Checksum
+			if checksum != (unique.Handle[pkg.Checksum]{}) {
+				checksumVal = checksum.Value()
+			}
+			t.Log(pkg.Encode(checksumVal)+":", v)
 		}
 	}
 }