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

input/size_test.go

@@ -1,16 +1,15 @@
 package input_test
 
 import (
-	"math/big"
 	"testing"
 
 	"github.com/btcsuite/btcd/blockchain"
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -37,12 +36,9 @@ var (
 	testPreimage = make([]byte, 32)
 
 	// testPubkey is a pubkey used in script size calculation.
-	testPubkey = &btcec.PublicKey{
-		X: &big.Int{},
-		Y: &big.Int{},
-	}
+	testPubkey = &btcec.PublicKey{}
 
-	testPrivkey, _ = btcec.PrivKeyFromBytes(btcec.S256(), make([]byte, 32))
+	testPrivkey, _ = btcec.PrivKeyFromBytes(make([]byte, 32))
 
 	testTx = wire.NewMsgTx(2)