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/chanfunding/psbt_assembler_test.go

@@ -10,12 +10,12 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/psbt"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -54,8 +54,8 @@ func TestPsbtIntent(t *testing.T) {
 	// negotiating with the remote peer and they accept. By accepting, they
 	// send over their multisig key that's going to be used for the funding
 	// output. With that known, we can start crafting a PSBT.
-	_, localPubkey := btcec.PrivKeyFromBytes(btcec.S256(), localPrivkey)
-	_, remotePubkey := btcec.PrivKeyFromBytes(btcec.S256(), remotePrivkey)
+	_, localPubkey := btcec.PrivKeyFromBytes(localPrivkey)
+	_, remotePubkey := btcec.PrivKeyFromBytes(remotePrivkey)
 	psbtIntent.BindKeys(
 		&keychain.KeyDescriptor{PubKey: localPubkey}, remotePubkey,
 	)
@@ -197,8 +197,8 @@ func TestPsbtIntentBasePsbt(t *testing.T) {
 
 	// Generate the funding multisig keys and the address so we can compare
 	// it to the output of the intent.
-	_, localPubkey := btcec.PrivKeyFromBytes(btcec.S256(), localPrivkey)
-	_, remotePubkey := btcec.PrivKeyFromBytes(btcec.S256(), remotePrivkey)
+	_, localPubkey := btcec.PrivKeyFromBytes(localPrivkey)
+	_, remotePubkey := btcec.PrivKeyFromBytes(remotePrivkey)
 	// Make sure the output script address is correct.
 	script, _, err := input.GenFundingPkScript(
 		localPubkey.SerializeCompressed(),
@@ -474,8 +474,8 @@ func TestPsbtVerify(t *testing.T) {
 	psbtIntent := intent.(*PsbtIntent)
 
 	// Bind our test keys to get the funding parameters.
-	_, localPubkey := btcec.PrivKeyFromBytes(btcec.S256(), localPrivkey)
-	_, remotePubkey := btcec.PrivKeyFromBytes(btcec.S256(), remotePrivkey)
+	_, localPubkey := btcec.PrivKeyFromBytes(localPrivkey)
+	_, remotePubkey := btcec.PrivKeyFromBytes(remotePrivkey)
 	psbtIntent.BindKeys(
 		&keychain.KeyDescriptor{PubKey: localPubkey}, remotePubkey,
 	)
@@ -642,8 +642,8 @@ func TestPsbtFinalize(t *testing.T) {
 	psbtIntent := intent.(*PsbtIntent)
 
 	// Bind our test keys to get the funding parameters.
-	_, localPubkey := btcec.PrivKeyFromBytes(btcec.S256(), localPrivkey)
-	_, remotePubkey := btcec.PrivKeyFromBytes(btcec.S256(), remotePrivkey)
+	_, localPubkey := btcec.PrivKeyFromBytes(localPrivkey)
+	_, remotePubkey := btcec.PrivKeyFromBytes(remotePrivkey)
 	psbtIntent.BindKeys(
 		&keychain.KeyDescriptor{PubKey: localPubkey}, remotePubkey,
 	)