multi: move Input interface and related code

This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.

invoices/invoiceregistry.go

@@ -4,11 +4,11 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"fmt"
-	"github.com/btcsuite/btcd/chaincfg"
 	"sync"
 	"sync/atomic"
 	"time"
 
+	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"