march: split src and dst

splits m.key into separate functions for src and dst to prepare for
lib/transform which will want to do transforms on the src side only.

Co-Authored-By: Nick Craig-Wood <nick@craig-wood.com>

fs/march/march_test.go

@@ -491,7 +491,11 @@ func TestMatchListings(t *testing.T) {
 			// Make a channel to send the source (0) or dest (1) using a list.Sorter
 			makeChan := func(offset int) <-chan fs.DirEntry {
 				out := make(chan fs.DirEntry)
-				ls, err := list.NewSorter(ctx, nil, list.SortToChan(out), m.key)
+				key := m.dstKey
+				if offset == 0 {
+					key = m.srcKey
+				}
+				ls, err := list.NewSorter(ctx, nil, list.SortToChan(out), key)
 				require.NoError(t, err)
 				wg.Add(1)
 				go func() {