### 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/LocalPhysicalPlanOptimizerTests.java
* Expects
     * LimitExec[500[INTEGER]]
     * \_AggregateExec[[],[COUNT([2a][KEYWORD]) AS c],FINAL,null]
     *   \_ExchangeExec[[count{r}#14, seen{r}#15],true]
     *     \_EsStatsQueryExec[test], stats[Stat[name=*, type=COUNT, query=null]]],
     *     query[{"esql_single_value":{"field":"emp_no","next":{"range":{"emp_no":{"gt":10040,"boost":1.0}}}}}][count{r}#30, seen{r}#31],
     *       limit[],
     */
    public void testCountAllWithFilter() {
        var plan = plan("from test | where emp_no > 10040 | stats c = count(*)");
        var stat = queryStatsFor(plan);
        assertThat(stat.type(), is(StatsType.COUNT));
        assertThat(stat.query(), is(nullValue()));
    }
    /**
     * Expects
     * LimitExec[500[INTEGER]]
     * \_AggregateExec[[],[COUNT(emp_no{f}#5) AS c],FINAL,null]
     *   \_ExchangeExec[[count{r}#15, seen{r}#16],true]
     *     \_EsStatsQueryExec[test], stats[Stat[name=emp_no, type=COUNT, query={
     *   "exists" : {
     *     "field" : "emp_no",
     *     "boost" : 1.0
     *   }
     * }]]], query[{"esql_single_value":{"field":"emp_no","next":{"range":{"emp_no":{"gt":10040,"boost":1.0}}}}}][count{r}#31, seen{r}#32],
     *   limit[],
     */
    public void testCountFieldWithFilter() {
        var plan = plan("from test | where emp_no > 10040 | stats c = count(emp_no)", IS_SV_STATS);
        var stat = queryStatsFor(plan);