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/integrated_routing_context_test.go

@@ -200,7 +200,7 @@ func (c *integratedRoutingContext) testPayment(maxParts uint32,
 	}
 
 	session, err := newPaymentSession(
-		&payment, getBandwidthHints,
+		&payment, c.graph.source.pubkey, getBandwidthHints,
 		func() (routingGraph, func(), error) {
 			return c.graph, func() {}, nil
 		},