multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

routing/route/route_test.go

@@ -5,14 +5,14 @@ import (
 	"encoding/hex"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
 )
 
 var (
 	testPrivKeyBytes, _ = hex.DecodeString("e126f68f7eafcc8b74f54d269fe206be715000f94dac067d1c04a8ca3b2db734")
-	_, testPubKey       = btcec.PrivKeyFromBytes(btcec.S256(), testPrivKeyBytes)
+	_, testPubKey       = btcec.PrivKeyFromBytes(testPrivKeyBytes)
 	testPubKeyBytes, _  = NewVertexFromBytes(testPubKey.SerializeCompressed())
 )