multi: move node ann validation code to netann pkg

The `netann` package is a more appropriate place for this code to live.
Also, once the funding transaction code is moved out of the
`graph.Builder`, then no `lnwire` validation will occur in the `graph`
package.

discovery/gossiper.go

@@ -1977,7 +1977,7 @@ func (d *AuthenticatedGossiper) fetchPKScript(chanID *lnwire.ShortChannelID) (
 func (d *AuthenticatedGossiper) addNode(msg *lnwire.NodeAnnouncement,
 	op ...batch.SchedulerOption) error {
 
-	if err := graph.ValidateNodeAnn(msg); err != nil {
+	if err := netann.ValidateNodeAnn(msg); err != nil {
 		return fmt.Errorf("unable to validate node announcement: %w",
 			err)
 	}