### 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
*/
    public void testExtractorMultiEvalWithSameName() {
        var plan = physicalPlan("""
            from test
            | eval emp_no = emp_no + 1
            | eval emp_no = emp_no + 1
            """);
        var optimized = optimizedPlan(plan);
        var eval = as(optimized, EvalExec.class);
        var topLimit = as(eval.child(), LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        assertThat(
            names(extract.attributesToExtract()),
            contains("_meta_field", "emp_no", "first_name", "gender", "job", "job.raw", "languages", "last_name", "long_noidx", "salary")
        );
    }
    public void testExtractorsOverridingFields() {
        var plan = physicalPlan("""
            from test
            | stats emp_no = sum(emp_no)
            """);
        var optimized = optimizedPlan(plan);
        var limit = as(optimized, LimitExec.class);
        var node = as(limit.child(), AggregateExec.class);
        var exchange = asRemoteExchange(node.child());
        var aggregate = as(exchange.child(), AggregateExec.class);
        var extract = as(aggregate.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), contains("emp_no"));