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_01_to_11/zpay32/decode.go

@@ -8,11 +8,12 @@ import (
 	"strings"
 	"time"
 
-	"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/bech32"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/bech32"
 	lnwire "github.com/lightningnetwork/lnd/channeldb/migration/lnwire21"
 )
 
@@ -112,8 +113,7 @@ func Decode(invoice string, net *chaincfg.Params) (*Invoice, error) {
 	} else {
 		headerByte := recoveryID + 27 + 4
 		compactSign := append([]byte{headerByte}, sig[:]...)
-		pubkey, _, err := btcec.RecoverCompact(btcec.S256(),
-			compactSign, hash)
+		pubkey, _, err := ecdsa.RecoverCompact(compactSign, hash)
 		if err != nil {
 			return nil, err
 		}
@@ -350,7 +350,7 @@ func parseDestination(data []byte) (*btcec.PublicKey, error) {
 		return nil, err
 	}
 
-	return btcec.ParsePubKey(base256Data, btcec.S256())
+	return btcec.ParsePubKey(base256Data)
 }
 
 // parseExpiry converts the data (encoded in base32) into the expiry time.
@@ -452,7 +452,7 @@ func parseRouteHint(data []byte) ([]HopHint, error) {
 
 	for len(base256Data) > 0 {
 		hopHint := HopHint{}
-		hopHint.NodeID, err = btcec.ParsePubKey(base256Data[:33], btcec.S256())
+		hopHint.NodeID, err = btcec.ParsePubKey(base256Data[:33])
 		if err != nil {
 			return nil, err
 		}

channeldb/migration_01_to_11/zpay32/hophint.go

@@ -1,6 +1,6 @@
 package zpay32
 
-import "github.com/btcsuite/btcd/btcec"
+import "github.com/btcsuite/btcd/btcec/v2"
 
 const (
 	// DefaultFinalCLTVDelta is the default value to be used as the final

channeldb/migration_01_to_11/zpay32/invoice.go

@@ -5,9 +5,9 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
-	"github.com/btcsuite/btcutil"
 	lnwire "github.com/lightningnetwork/lnd/channeldb/migration/lnwire21"
 )
 
@@ -103,7 +103,7 @@ type MessageSigner struct {
 	// SignCompact signs the passed hash with the node's privkey. The
 	// returned signature should be 65 bytes, where the last 64 are the
 	// compact signature, and the first one is a header byte. This is the
-	// format returned by btcec.SignCompact.
+	// format returned by ecdsa.SignCompact.
 	SignCompact func(hash []byte) ([]byte, error)
 }