### 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
LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | rename salary as l
            | keep l, emp_no, first_name
            | sort l
            | limit 100
            | sort first_name
            | where l > 1
            | sort emp_no""");
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);
        assertThat(orderNames(topN), contains("emp_no", "first_name"));
        var filter = as(topN.child(), Filter.class);
        var topN2 = as(filter.child(), TopN.class);
        assertThat(orderNames(topN2), contains("salary", "emp_no"));
        as(topN2.child(), EsRelation.class);
    }
    public void testDontPruneSameFieldDifferentDirectionSortClauses() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort salary nulls last, emp_no desc nulls first
            | where salary > 2
            | eval e = emp_no * 2
            | keep salary, emp_no, e
            | sort e, emp_no, salary desc, emp_no desc""");
        var keep = as(plan, Project.class);
        var topN = as(keep.child(), TopN.class);