### 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(orderNames(topN), contains("salary", "emp_no"));
        as(topN.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughEval() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | eval x = salary + 1
            | sort x""");
        var topN = as(plan, TopN.class);
        assertThat(orderNames(topN), contains("x", "emp_no"));
        var eval = as(topN.child(), Eval.class);
        as(eval.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughEvalWithTwoDefs() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | sort emp_no
            | eval x = salary + 1, y = salary + 2
            | eval z = x * y
            | sort z""");
        var topN = as(plan, TopN.class);
        assertThat(orderNames(topN), contains("z", "emp_no"));
        var eval = as(topN.child(), Eval.class);
        assertThat(Expressions.names(eval.fields()), contains("x", "y", "z"));
        as(eval.child(), EsRelation.class);
    }
    public void testCombineOrderByThroughDissect() {
        LogicalPlan plan = optimizedPlan("""
            from test