routing: remove sourceNode from routingGraph interface

In this commit, we further reduce the routingGraph interface and this
time we make it more node-agnostic so that it can be backed by any graph
and not one with a concept of "sourceNode".

routing/pathfind_test.go

@@ -3218,7 +3218,8 @@ func dbFindPath(graph *channeldb.ChannelGraph,
 			bandwidthHints:  bandwidthHints,
 			graph:           routingGraph,
 		},
-		r, cfg, source, target, amt, timePref, finalHtlcExpiry,
+		r, cfg, sourceNode.PubKeyBytes, source, target, amt, timePref,
+		finalHtlcExpiry,
 	)
 
 	return route, err