### 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
assertThat(
            topN.order(),
            contains(
                new Order(
                    EMPTY,
                    new ReferenceAttribute(EMPTY, "e", INTEGER, null, Nullability.TRUE, null, false),
                    Order.OrderDirection.ASC,
                    Order.NullsPosition.LAST
                ),
                new Order(
                    EMPTY,
                    new FieldAttribute(EMPTY, "emp_no", mapping.get("emp_no")),
                    Order.OrderDirection.ASC,
                    Order.NullsPosition.LAST
                ),
                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.FIRST
                ),
                new Order(
                    EMPTY,
                    new FieldAttribute(EMPTY, "salary", mapping.get("salary")),