sweep: deepen the interface Aggregator

This commit makes the `ClusterInputs` directly returning the `InputSet`
so the sweeper doesn't know about the existence of `Cluster` interface.
This way we can have a deeper interface as the sweeper only needs to
interact with `Aggregator` only to get the final input sets, leaving the
implementation details being managed by `SimpleAggregator` and future
aggregators.

sweep/aggregator_test.go

@@ -320,7 +320,7 @@ func TestClusterByLockTime(t *testing.T) {
 	)
 
 	// Create a test aggregator.
-	s := NewSimpleUtxoAggregator(nil, maxFeeRate)
+	s := NewSimpleUtxoAggregator(nil, maxFeeRate, 100)
 
 	testCases := []struct {
 		name string