refactored overlay class

overlay.GetOverlay(name) returns now an error if the overlay doesn't
exist. This is the most canonical way to act if there is no overlay.

internal/app/wwctl/overlay/imprt/root.go

@@ -24,15 +24,13 @@ var (
 			}
 		},
 	}
-	OverwriteFile   bool
-	NoOverlayUpdate bool
-	CreateDirs      bool
-	Workers         int
+	OverwriteFile bool
+	CreateDirs    bool
+	Workers       int
 )
 
 func init() {
 	baseCmd.PersistentFlags().BoolVarP(&OverwriteFile, "overwrite", "o", false, "Overwrite file if exists")
-	baseCmd.PersistentFlags().BoolVarP(&NoOverlayUpdate, "noupdate", "n", false, "Don't update overlays")
 	baseCmd.PersistentFlags().BoolVarP(&CreateDirs, "parents", "p", false, "Create any necessary parent directories")
 	baseCmd.PersistentFlags().IntVar(&Workers, "workers", 0, "The number of parallel workers building overlays (<=0 indicates 1 worker per CPU)")
 }