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

watchtower/lookout/justice_descriptor.go

@@ -4,11 +4,11 @@ import (
 	"errors"
 
 	"github.com/btcsuite/btcd/blockchain"
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/txsort"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/txsort"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/watchtower/blob"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
@@ -121,7 +121,7 @@ func (p *JusticeDescriptor) commitToRemoteInput() (*breachedInput, error) {
 	} else {
 		// Since the to-remote witness script should just be a regular p2wkh
 		// output, we'll parse it to retrieve the public key.
-		toRemotePubKey, err := btcec.ParsePubKey(toRemoteScript, btcec.S256())
+		toRemotePubKey, err := btcec.ParsePubKey(toRemoteScript)
 		if err != nil {
 			return nil, err
 		}

watchtower/lookout/justice_descriptor_test.go

@@ -5,11 +5,11 @@ import (
 	"time"
 
 	"github.com/btcsuite/btcd/blockchain"
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/txsort"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/txsort"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -93,13 +93,13 @@ func testJusticeDescriptor(t *testing.T, blobType blob.Type) {
 
 	// Parse the key pairs for all keys used in the test.
 	revSK, revPK := btcec.PrivKeyFromBytes(
-		btcec.S256(), revPrivBytes,
+		revPrivBytes,
 	)
 	_, toLocalPK := btcec.PrivKeyFromBytes(
-		btcec.S256(), toLocalPrivBytes,
+		toLocalPrivBytes,
 	)
 	toRemoteSK, toRemotePK := btcec.PrivKeyFromBytes(
-		btcec.S256(), toRemotePrivBytes,
+		toRemotePrivBytes,
 	)
 
 	// Create the signer, and add the revocation and to-remote privkeys.