Simplify the implementation.

base/src/main/java/io/spine/reflect/PackageGraph.java

@@ -137,9 +137,8 @@ private static Graph<PackageInfo> buildGraph(List<Package> packages) {
                                 .nodeOrder(ElementOrder.<PackageInfo>natural())
                                 .build();
         Queue<Package> queue = new ArrayDeque<>(packages);
-        var first = queue.poll();
-        while (first != null) {
-            final var current = first;
+        var current = queue.poll();
+        while (current != null) {
             var isDirectParent = IsDirectParent.of(current);
             var directParent = graph.nodes().stream()
                     .filter((node) -> isDirectParent.test(node.getValue()))
@@ -150,7 +149,7 @@ private static Graph<PackageInfo> buildGraph(List<Package> packages) {
             } else {
                 graph.addNode(newNode);
             }
-            first = queue.poll();
+            current = queue.poll();
         }
         return graph;
     }