### 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
)
        );
    }
    /**
     * Expects
     * ProjectExec[[x{r}#3]]
     * \_EvalExec[[1[INTEGER] AS x]]
     *   \_LimitExec[10000[INTEGER]]
     *     \_ExchangeExec[[],false]
     *       \_ProjectExec[[&lt;all-fields-projected&gt;{r}#12]]
     *         \_EvalExec[[null[NULL] AS &lt;all-fields-projected&gt;]]
     *           \_EsQueryExec[test], query[{"esql_single_value":{"field":"emp_no","next":{"range":{"emp_no":{"gt":10,"boost":1.0}}}}}]
     *            [_doc{f}#13], limit[10000], sort[] estimatedRowSize[8]
     */
    public void testProjectAllFieldsWhenOnlyTheCountMatters() {
        var plan = optimizedPlan(physicalPlan("""
            from test
            | where emp_no > 10
            | eval x = 1
            | keep x
            """));
        var project = as(plan, ProjectExec.class);
        var eval = as(project.child(), EvalExec.class);
        var limit = as(eval.child(), LimitExec.class);
        var exchange = as(limit.child(), ExchangeExec.class);
        var nullField = "<all-fields-projected>";
        project = as(exchange.child(), ProjectExec.class);
        assertThat(Expressions.names(project.projections()), contains(nullField));
        eval = as(project.child(), EvalExec.class);
        assertThat(Expressions.names(eval.fields()), contains(nullField));