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/test/test_interface.go

@@ -17,8 +17,9 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcjson"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/integration/rpctest"
@@ -26,7 +27,6 @@ import (
 	"github.com/btcsuite/btcd/rpcclient"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/btcsuite/btcwallet/chain"
 	"github.com/btcsuite/btcwallet/walletdb"
 	_ "github.com/btcsuite/btcwallet/walletdb/bdb"
@@ -81,8 +81,8 @@ var (
 	netParams = &chaincfg.RegressionNetParams
 	chainHash = netParams.GenesisHash
 
-	_, alicePub = btcec.PrivKeyFromBytes(btcec.S256(), testHdSeed[:])
-	_, bobPub   = btcec.PrivKeyFromBytes(btcec.S256(), bobsPrivKey)
+	_, alicePub = btcec.PrivKeyFromBytes(testHdSeed[:])
+	_, bobPub   = btcec.PrivKeyFromBytes(bobsPrivKey)
 
 	// The number of confirmations required to consider any created channel
 	// open.
@@ -2001,8 +2001,7 @@ func testSignOutputUsingTweaks(r *rpctest.Harness,
 	// we'll generate a commitment pre-image, then derive a revocation key
 	// and single tweak from that.
 	commitPreimage := bytes.Repeat([]byte{2}, 32)
-	commitSecret, commitPoint := btcec.PrivKeyFromBytes(btcec.S256(),
-		commitPreimage)
+	commitSecret, commitPoint := btcec.PrivKeyFromBytes(commitPreimage)
 
 	revocationKey := input.DeriveRevocationPubkey(pubKey.PubKey, commitPoint)
 	commitTweak := input.SingleTweakBytes(commitPoint, pubKey.PubKey)