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

lnwallet/btcwallet/signer.go

@@ -3,15 +3,15 @@ package btcwallet
 import (
 	"fmt"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/hdkeychain"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/hdkeychain"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/btcsuite/btcwallet/walletdb"
-	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
@@ -367,7 +367,7 @@ func (b *BtcWallet) SignOutputRaw(tx *wire.MsgTx,
 	}
 
 	// Chop off the sighash flag at the end of the signature.
-	return btcec.ParseDERSignature(sig[:len(sig)-1], btcec.S256())
+	return ecdsa.ParseDERSignature(sig[:len(sig)-1])
 }
 
 // ComputeInputScript generates a complete InputScript for the passed
@@ -412,7 +412,7 @@ var _ input.Signer = (*BtcWallet)(nil)
 //
 // NOTE: This is a part of the MessageSigner interface.
 func (b *BtcWallet) SignMessage(keyLoc keychain.KeyLocator,
-	msg []byte, doubleHash bool) (*btcec.Signature, error) {
+	msg []byte, doubleHash bool) (*ecdsa.Signature, error) {
 
 	// First attempt to fetch the private key which corresponds to the
 	// specified public key.
@@ -430,12 +430,7 @@ func (b *BtcWallet) SignMessage(keyLoc keychain.KeyLocator,
 	} else {
 		msgDigest = chainhash.HashB(msg)
 	}
-	sign, err := privKey.Sign(msgDigest)
-	if err != nil {
-		return nil, errors.Errorf("unable sign the message: %v", err)
-	}
-
-	return sign, nil
+	return ecdsa.Sign(privKey, msgDigest), nil
 }
 
 // A compile time check to ensure that BtcWallet implements the MessageSigner