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

keychain/interface_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcwallet/snacl"
@@ -376,7 +376,7 @@ func TestSecretKeyRingDerivation(t *testing.T) {
 				// We'll try again, but this time with an
 				// unknown public key.
 				_, pub := btcec.PrivKeyFromBytes(
-					btcec.S256(), testHDSeed[:],
+					testHDSeed[:],
 				)
 				keyDesc.PubKey = pub