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

channeldb/migration/lnwire21/accept_channel.go

@@ -3,8 +3,8 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 )
 
 // AcceptChannel is the message Bob sends to Alice after she initiates the

channeldb/migration/lnwire21/channel_reestablish.go

@@ -3,7 +3,7 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 // ChannelReestablish is a message sent between peers that have an existing

channeldb/migration/lnwire21/closing_signed.go

@@ -3,7 +3,7 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcutil"
 )
 
 // ClosingSigned is sent by both parties to a channel once the channel is clear

channeldb/migration/lnwire21/funding_locked.go

@@ -3,7 +3,7 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 // FundingLocked is the message that both parties to a new channel creation

channeldb/migration/lnwire21/lnwire.go

@@ -9,10 +9,10 @@ import (
 	"math"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/tor"
 )
@@ -529,7 +529,7 @@ func ReadElement(r io.Reader, element interface{}) error {
 			return err
 		}
 
-		pubKey, err := btcec.ParsePubKey(b[:], btcec.S256())
+		pubKey, err := btcec.ParsePubKey(b[:])
 		if err != nil {
 			return err
 		}

channeldb/migration/lnwire21/msat.go

@@ -3,7 +3,7 @@ package lnwire
 import (
 	"fmt"
 
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcutil"
 )
 
 const (

channeldb/migration/lnwire21/netaddress.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
 )
 

channeldb/migration/lnwire21/open_channel.go

@@ -3,9 +3,9 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
-	"github.com/btcsuite/btcutil"
 )
 
 // FundingFlag represents the possible bit mask values for the ChannelFlags

channeldb/migration/lnwire21/revoke_and_ack.go

@@ -3,7 +3,7 @@ package lnwire
 import (
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 // RevokeAndAck is sent by either side once a CommitSig message has been

channeldb/migration/lnwire21/signature.go

@@ -3,14 +3,14 @@ package lnwire
 import (
 	"fmt"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/lightningnetwork/lnd/input"
 )
 
 // Sig is a fixed-sized ECDSA signature. Unlike Bitcoin, we use fixed sized
 // signatures on the wire, instead of DER encoded signatures. This type
 // provides several methods to convert to/from a regular Bitcoin DER encoded
-// signature (raw bytes and *btcec.Signature).
+// signature (raw bytes and *ecdsa.Signature).
 type Sig [64]byte
 
 // NewSigFromRawSignature returns a Sig from a Bitcoin raw signature encoded in
@@ -27,7 +27,7 @@ func NewSigFromRawSignature(sig []byte) (Sig, error) {
 	// which means the length of R is the 4th byte and the length of S
 	// is the second byte after R ends. 0x02 signifies a length-prefixed,
 	// zero-padded, big-endian bigint. 0x30 signifies a DER signature.
-	// See the Serialize() method for btcec.Signature for details.
+	// See the Serialize() method for ecdsa.Signature for details.
 	rLen := sig[3]
 	sLen := sig[5+rLen]
 
@@ -64,7 +64,7 @@ func NewSigFromRawSignature(sig []byte) (Sig, error) {
 }
 
 // NewSigFromSignature creates a new signature as used on the wire, from an
-// existing btcec.Signature.
+// existing ecdsa.Signature.
 func NewSigFromSignature(e input.Signature) (Sig, error) {
 	if e == nil {
 		return Sig{}, fmt.Errorf("cannot decode empty signature")
@@ -74,12 +74,12 @@ func NewSigFromSignature(e input.Signature) (Sig, error) {
 	return NewSigFromRawSignature(e.Serialize())
 }
 
-// ToSignature converts the fixed-sized signature to a btcec.Signature objects
+// ToSignature converts the fixed-sized signature to a ecdsa.Signature objects
 // which can be used for signature validation checks.
-func (b *Sig) ToSignature() (*btcec.Signature, error) {
+func (b *Sig) ToSignature() (*ecdsa.Signature, error) {
 	// Parse the signature with strict checks.
 	sigBytes := b.ToSignatureBytes()
-	sig, err := btcec.ParseDERSignature(sigBytes, btcec.S256())
+	sig, err := ecdsa.ParseDERSignature(sigBytes)
 	if err != nil {
 		return nil, err
 	}