### 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
* Expected
     * ProjectExec[[languages{f}#10, salary{f}#12, x{r}#6]]
     * \_EvalExec[[languages{f}#10 + 1[INTEGER] AS x]]
     *   \_TopNExec[[Order[salary{f}#12,ASC,LAST]],1[INTEGER]]
     *     \_ExchangeExec[]
     *       \_ProjectExec[[languages{f}#10, salary{f}#12]]
     *         \_FieldExtractExec[languages{f}#10]
     *           \_EsQueryExec[test], query[][_doc{f}#14], limit[1], sort[[salary]]
     */
    public void testDoNotAliasesDefinedAfterTheExchange() throws Exception {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | sort salary
            | limit 1
            | keep languages, salary
            | eval x = languages + 1
            """));
        var project = as(optimized, ProjectExec.class);
        var eval = as(project.child(), EvalExec.class);
        var topN = as(eval.child(), TopNExec.class);
        var exchange = asRemoteExchange(topN.child());
        project = as(exchange.child(), ProjectExec.class);
        assertThat(names(project.projections()), contains("languages", "salary"));
        var extract = as(project.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), contains("languages", "salary"));
        var source = source(extract.child());
        assertThat(source.limit(), is(topN.limit()));
        assertThat(source.sorts(), is(sorts(topN.order())));
        assertThat(source.limit(), is(l(1)));
        assertThat(source.sorts().size(), is(1));