### 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
| where _index != "test"
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(limit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        var source = source(extract.child());
        var bq = as(source.query(), BoolQueryBuilder.class);
        assertThat(bq.mustNot().size(), is(1));
        var tq = as(bq.mustNot().get(0), TermQueryBuilder.class);
        assertThat(tq.fieldName(), is("_index"));
        assertThat(tq.value(), is("test"));
    }
    /*
     * LimitExec[10000[INTEGER]]
     * \_ExchangeExec[[],false]
     *   \_ProjectExec[[_meta_field{f}#9, emp_no{f}#3, first_name{f}#4, gender{f}#5, languages{f}#6, last_name{f}#7, salary{f}#8, _in
     *     dex{m}#1]]
     *     \_FieldExtractExec[_meta_field{f}#9, emp_no{f}#3, first_name{f}#4, gen..]
     *       \_LimitExec[10000[INTEGER]]
     *         \_FilterExec[_index{m}#1 > [74 65 73 74][KEYWORD]]
     *           \_FieldExtractExec[_index{m}#1]
     *             \_EsQueryExec[test], query[][_doc{f}#10], limit[], sort[] estimatedRowSize[266]
     */
    public void testDontPushDownMetadataIndexInInequality() {
        for (var t : List.of(
            tuple(">", GreaterThan.class),
            tuple(">=", GreaterThanOrEqual.class),
            tuple("<", LessThan.class),