Exclude `project.yml` as rev-depgraph node key

This commit changes the debug log output.

Nothing ever depends on `project.yml`, so don't include it as a node key
in reverse dependency graphs.  This change removes some zero-information
lines from the debug output.
diff --git a/newt/deprepo/graph.go b/newt/deprepo/graph.go
index e482562..557b2ce 100644
--- a/newt/deprepo/graph.go
+++ b/newt/deprepo/graph.go
@@ -192,11 +192,14 @@
 
 	for dependent, nodes := range dg {
 		for _, node := range nodes {
-			rg[node.Name] = append(rg[node.Name], RevdepGraphNode{
-				Name:    dependent.Name,
-				Ver:     dependent.Ver,
-				VerReqs: node.VerReqs,
-			})
+			// Nothing depends on project.yml (""), so exclude it from the result.
+			if node.Name != "" {
+				rg[node.Name] = append(rg[node.Name], RevdepGraphNode{
+					Name:    dependent.Name,
+					Ver:     dependent.Ver,
+					VerReqs: node.VerReqs,
+				})
+			}
 		}
 	}