lnwallet+sweep: extend the WitnessGenerator type to return an InputScript

In this commit, we extend the WitnessGenerator type to now return an
InputScript. This allows it to be more encompassing, as now callers can
expect a sigScript to be populated if the input being swept requires a
sigScript field.

Along the way, we've also renamed input.BuildWitness to
input.CraftInputScript.  We also take a step towards allowing the
sweeper to sweep transactions for n2pwkh outputs. We do so by modifying
the BuiltWitness method to instead return an InputScript. Additionally,
when populating inputs if a sigScript is present, it will now be
populated.

breacharbiter.go

@@ -807,13 +807,13 @@ func (bo *breachedOutput) SignDesc() *lnwallet.SignDescriptor {
 	return &bo.signDesc
 }
 
-// BuildWitness computes a valid witness that allows us to spend from the
+// CraftInputScript computes a valid witness that allows us to spend from the
 // breached output. It does so by first generating and memoizing the witness
 // generation function, which parameterized primarily by the witness type and
 // sign descriptor. The method then returns the witness computed by invoking
 // this function on the first and subsequent calls.
-func (bo *breachedOutput) BuildWitness(signer lnwallet.Signer, txn *wire.MsgTx,
-	hashCache *txscript.TxSigHashes, txinIdx int) ([][]byte, error) {
+func (bo *breachedOutput) CraftInputScript(signer lnwallet.Signer, txn *wire.MsgTx,
+	hashCache *txscript.TxSigHashes, txinIdx int) (*lnwallet.InputScript, error) {
 
 	// First, we ensure that the witness generation function has been
 	// initialized for this breached output.
@@ -1082,15 +1082,16 @@ func (b *breachArbiter) sweepSpendableOutputsTxn(txWeight int64,
 		// First, we construct a valid witness for this outpoint and
 		// transaction using the SpendableOutput's witness generation
 		// function.
-		witness, err := so.BuildWitness(b.cfg.Signer, txn, hashCache,
-			idx)
+		inputScript, err := so.CraftInputScript(
+			b.cfg.Signer, txn, hashCache, idx,
+		)
 		if err != nil {
 			return err
 		}
 
 		// Then, we add the witness to the transaction at the
 		// appropriate txin index.
-		txn.TxIn[idx].Witness = witness
+		txn.TxIn[idx].Witness = inputScript.Witness
 
 		return nil
 	}