### 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
limit = as(aggPartial.child(), LimitExec.class);
        assertThat(limit.limit(), instanceOf(Literal.class));
        assertThat(limit.limit().fold(), equalTo(10));
        var exchange = as(limit.child(), ExchangeExec.class);
        project = as(exchange.child(), ProjectExec.class);
        var expectedFields = List.of("salary");
        assertThat(Expressions.names(project.projections()), is(expectedFields));
        var fieldExtract = as(project.child(), FieldExtractExec.class);
        assertThat(Expressions.names(fieldExtract.attributesToExtract()), is(expectedFields));
        var source = source(fieldExtract.child());
        assertThat(source.limit().fold(), equalTo(10));
    }
    private static EsQueryExec source(PhysicalPlan plan) {
        if (plan instanceof ExchangeExec exchange) {
            plan = exchange.child();
        }
        return as(plan, EsQueryExec.class);
    }
    private PhysicalPlan optimizedPlan(PhysicalPlan plan) {
        return optimizedPlan(plan, EsqlTestUtils.TEST_SEARCH_STATS);
    }
    private PhysicalPlan optimizedPlan(PhysicalPlan plan, SearchStats searchStats) {
        
        var p = EstimatesRowSize.estimateRowSize(0, physicalPlanOptimizer.optimize(plan));
        
        
        
        
        var l = p.transformUp(FragmentExec.class, fragment -> {
            var localPlan = PlannerUtils.localPlan(config, fragment, searchStats);