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

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"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -121,8 +122,7 @@ 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
 		}
@@ -359,7 +359,7 @@ 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.
@@ -461,7 +461,7 @@ 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
 		}