new: pkg supports 'deepin' (#306)

lib/data/model/pkg/manager.dart

@@ -105,6 +105,7 @@ enum PkgManager {
       case Dist.ubuntu:
       case Dist.kali:
       case Dist.armbian:
+      case Dist.deepin:
         return PkgManager.apt;
       case Dist.opensuse:
         return PkgManager.zypper;
@@ -114,7 +115,7 @@ enum PkgManager {
         return PkgManager.pacman;
       case Dist.alpine:
         return PkgManager.apk;
-      default:
+      case null:
         return null;
     }
   }