### 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
*
     * ProjectExec[[emp_no{f}#7, x{r}#4]]
     * \_TopNExec[[Order[emp_no{f}#7,ASC,LAST]],5[INTEGER]]
     *   \_ExchangeExec[]
     *     \_ProjectExec[[emp_no{f}#7, x{r}#4]]
     *       \_TopNExec[[Order[emp_no{f}#7,ASC,LAST]],5[INTEGER]]
     *         \_FieldExtractExec[emp_no{f}#7]
     *           \_EvalExec[[first_name{f}#8 AS x]]
     *             \_FieldExtractExec[first_name{f}#8]
     *               \_EsQueryExec[test], query[][_doc{f}#14], limit[]
     */
    public void testLocalProjectIncludeLocalAlias() throws Exception {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | sort emp_no
            | eval x = first_name
            | keep emp_no, x
            | limit 5
            """));
        var project = as(optimized, ProjectExec.class);
        var topN = as(project.child(), TopNExec.class);
        var exchange = asRemoteExchange(topN.child());
        project = as(exchange.child(), ProjectExec.class);
        assertThat(names(project.projections()), contains("emp_no", "x"));
        topN = as(project.child(), TopNExec.class);
        var extract = as(topN.child(), FieldExtractExec.class);
        var eval = as(extract.child(), EvalExec.class);
        extract = as(eval.child(), FieldExtractExec.class);
    }
    /**