Step aliases used inside a union are being dropped

[allenhadden]

This test illustrates the problem:

    @Test
    public void testAliasesWithinUnion() {
        Vertex a = this.sqlgGraph.addVertex(T.label, "A", "name", "A");
        Vertex b = this.sqlgGraph.addVertex(T.label, "B", "name", "B");

        a.addEdge("edge", b);

        // First, confirm that a simple traversal using an alias works properly.  This works as
        // expected (the failure is below with the union where the exact traversal is used inside the union).
        List<Vertex> noUnionItems = this.sqlgGraph.traversal()
                .V()
                .hasLabel("A")
                .as("alias1")
                .out()
                .<Vertex>select("alias1")
                .toList();

        Assert.assertEquals(1, noUnionItems.size());
        Assert.assertEquals(a, noUnionItems.get(0));

        // This one doesn't work even though the exact same traversal is used inside the union.  Debugging
        // the code shows that it cannot find the "alias1" label in the SelectOneStep (in the map method).
        List<Vertex> unionItems = this.sqlgGraph.traversal()
                .V()
                .limit(1)       // Normally an inject would be used here, but see #415
                .<Vertex>union(
                        __.V().hasLabel("A").as("alias1").out().select("alias1")
                )
                .toList();

        // This fails because unionItems contains 0 results.
        Assert.assertEquals(1, unionItems.size());
    }