### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
Order.NullsPosition.FIRST
                ),
                new Order(
                    EMPTY,
                    new FieldAttribute(EMPTY, "salary", mapping.get("salary")),
                    Order.OrderDirection.DESC,
                    Order.NullsPosition.FIRST
                ),
                new Order(
                    EMPTY,
                    new FieldAttribute(EMPTY, "emp_no", mapping.get("emp_no")),
                    Order.OrderDirection.DESC,
                    Order.NullsPosition.LAST
                ),
                new Order(
                    EMPTY,
                    new FieldAttribute(EMPTY, "salary", mapping.get("salary")),
                    Order.OrderDirection.DESC,
                    Order.NullsPosition.LAST
                )
            )
        );
        assertThat(topN.child().collect(OrderBy.class::isInstance), is(emptyList()));
    }
    public void testDontPruneSameFieldDifferentDirectionSortClauses_UsingAlias() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no desc
            | rename emp_no as e
            | keep e