routing: swap out final hop blinded route pub keys

If multiple blinded paths are provided, they will each have a different
pub key for the destination node. This makes using our existing
pathfinding logic tricky since it depends on having a single destination
node (characterised by a single pub key). We want to re-use this logic.
So what we do is swap out the pub keys of the destinaion hop with a
pseudo target pub key. This will then be used during pathfinding. Later
on once a path is found, we will swap the real destination keys back in
so that onion creation can be done.

routing/router.go

@@ -573,20 +573,7 @@ func getTargetNode(target *route.Vertex,
 		return route.Vertex{}, ErrTargetAndBlinded
 
 	case blinded:
-		blindedPayment := blindedPathSet.GetPath()
-
-		// If we're dealing with an edge-case blinded path that just
-		// has an introduction node (first hop expected to be the intro
-		// hop), then we return the unblinded introduction node as our
-		// target.
-		hops := blindedPayment.BlindedPath.BlindedHops
-		if len(hops) == 1 {
-			return route.NewVertex(
-				blindedPayment.BlindedPath.IntroductionPoint,
-			), nil
-		}
-
-		return route.NewVertex(hops[len(hops)-1].BlindedNodePub), nil
+		return route.NewVertex(blindedPathSet.TargetPubKey()), nil
 
 	case targetSet:
 		return *target, nil