internal/pkg: deduplicate dependency errors

This significantly simplifies error reporting for caller.

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

internal/pkg/exec_test.go

@@ -11,6 +11,7 @@ import (
 	"os/exec"
 	"slices"
 	"testing"
+	"unique"
 
 	"hakurei.app/container/check"
 	"hakurei.app/container/stub"
@@ -74,7 +75,14 @@ func TestExec(t *testing.T) {
 							return stub.UniqueError(0xcafe)
 						},
 					}),
-				), nil, pkg.Checksum{}, errors.Join(stub.UniqueError(0xcafe))},
+				), nil, pkg.Checksum{}, &pkg.DependencyCureError{
+					{
+						Ident: unique.Make(pkg.ID(pkg.MustDecode(
+							"CWEoJqnSBpWf8uryC2qnIe3O1a_FZWUWZGbiVPsQFGW7pvDHiSwoK3QCU9-uxN87",
+						))),
+						Err: stub.UniqueError(0xcafe),
+					},
+				}},
 
 				{"invalid paths", pkg.NewExec(
 					"", nil, 0,