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

lntest/mock/signer.go

@@ -3,7 +3,8 @@ package mock
 import (
 	"fmt"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
@@ -82,7 +83,7 @@ func (s *SingleSigner) SignOutputRaw(tx *wire.MsgTx,
 		return nil, err
 	}
 
-	return btcec.ParseDERSignature(sig[:len(sig)-1], btcec.S256())
+	return ecdsa.ParseDERSignature(sig[:len(sig)-1])
 }
 
 // ComputeInputScript computes an input script with the stored private key
@@ -116,7 +117,7 @@ func (s *SingleSigner) ComputeInputScript(tx *wire.MsgTx,
 // SignMessage takes a public key and a message and only signs the message
 // with the stored private key if the public key matches the private key.
 func (s *SingleSigner) SignMessage(keyLoc keychain.KeyLocator,
-	msg []byte, doubleHash bool) (*btcec.Signature, error) {
+	msg []byte, doubleHash bool) (*ecdsa.Signature, error) {
 
 	mockKeyLoc := s.KeyLoc
 	if s.KeyLoc.IsEmpty() {
@@ -133,10 +134,5 @@ func (s *SingleSigner) SignMessage(keyLoc keychain.KeyLocator,
 	} else {
 		digest = chainhash.HashB(msg)
 	}
-	sign, err := s.Privkey.Sign(digest)
-	if err != nil {
-		return nil, fmt.Errorf("can't sign the message: %v", err)
-	}
-
-	return sign, nil
+	return ecdsa.Sign(s.Privkey, digest), nil
 }