### 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/PhysicalPlanOptimizerTests.java
var source = source(eval.child());
    }
    /**
     * ProjectExec[[a{r}#5]]
     * \_EvalExec[[__a_SUM@81823521{r}#15 / __a_COUNT@31645621{r}#16 AS a]]
     *   \_LimitExec[10000[INTEGER]]
     *     \_AggregateExec[[],[SUM(salary{f}#11) AS __a_SUM@81823521, COUNT(salary{f}#11) AS __a_COUNT@31645621],FINAL,24]
     *       \_AggregateExec[[],[SUM(salary{f}#11) AS __a_SUM@81823521, COUNT(salary{f}#11) AS __a_COUNT@31645621],PARTIAL,16]
     *         \_LimitExec[10[INTEGER]]
     *           \_ExchangeExec[[],false]
     *             \_ProjectExec[[salary{f}#11]]
     *               \_FieldExtractExec[salary{f}#11]
     *                 \_EsQueryExec[test], query[][_doc{f}#17], limit[10], sort[] estimatedRowSize[8]
     */
    public void testAvgSurrogateFunctionAfterRenameAndLimit() {
        var plan = optimizedPlan(physicalPlan("""
            from test
            | limit 10
            | rename first_name as FN
            | stats a = avg(salary)
            """));
        var project = as(plan, ProjectExec.class);
        var eval = as(project.child(), EvalExec.class);
        var limit = as(eval.child(), LimitExec.class);
        assertThat(limit.limit(), instanceOf(Literal.class));
        assertThat(limit.limit().fold(), equalTo(10000));
        var aggFinal = as(limit.child(), AggregateExec.class);
        assertThat(aggFinal.getMode(), equalTo(FINAL));
        var aggPartial = as(aggFinal.child(), AggregateExec.class);
        assertThat(aggPartial.getMode(), equalTo(AggregateExec.Mode.PARTIAL));