htlcswitch: move hop iterator into htlcswitch/hop package

Prepares for onion blob decoding outside of htlcswitch.

htlcswitch/test_utils.go

@@ -1054,7 +1054,7 @@ func createTwoClusterChannels(aliceToBob, bobToCarol btcutil.Amount) (
 type hopNetwork struct {
 	feeEstimator *mockFeeEstimator
 	globalPolicy ForwardingPolicy
-	obfuscator   ErrorEncrypter
+	obfuscator   hop.ErrorEncrypter
 
 	defaultDelta uint32
 }
@@ -1101,7 +1101,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 			ForwardPackets:     server.htlcSwitch.ForwardPackets,
 			DecodeHopIterators: decoder.DecodeHopIterators,
 			ExtractErrorEncrypter: func(*btcec.PublicKey) (
-				ErrorEncrypter, lnwire.FailCode) {
+				hop.ErrorEncrypter, lnwire.FailCode) {
 				return h.obfuscator, lnwire.CodeNone
 			},
 			FetchLastChannelUpdate: mockGetChanUpdateMessage,