Rename and refactor overlay constructor functions

- Reduces redundancy
- Adds clarity

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/app/wwctl/overlay/delete/main.go

@@ -22,7 +22,7 @@ func CobraRunE(cmd *cobra.Command, args []string) error {
 		fileName = args[1]
 	}
 
-	overlayPath, isSite := overlay.OverlaySourceDir(overlayName)
+	overlayPath, isSite := overlay.GetOverlay(overlayName)
 	if !isSite {
 		return fmt.Errorf("distribution overlay can't deleted")