### 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
var optimized = optimizedPlan(plan);
        var topLimit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extractRest = as(project.child(), FieldExtractExec.class);
        var limit = as(extractRest.child(), LimitExec.class);
        var filter = as(limit.child(), FilterExec.class);
        var extract = as(filter.child(), FieldExtractExec.class);
        var source = source(extract.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        var gt = as(filter.condition(), GreaterThan.class);
        as(gt.left(), Round.class);
        var rq = as(sv(source.query(), "salary"), RangeQueryBuilder.class);
        assertThat(rq.fieldName(), equalTo("salary"));
        assertThat(rq.to(), equalTo(10));
        assertThat(rq.includeLower(), equalTo(false));
        assertThat(rq.from(), nullValue());
    }
    public void testNoPushDownNonFoldableInComparisonFilter() {
        var plan = physicalPlan("""
            from test
            | where emp_no > salary
            """);
        var optimized = optimizedPlan(plan);
        var topLimit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extractRest = as(project.child(), FieldExtractExec.class);
        var limit = as(extractRest.child(), LimitExec.class);
        var filter = as(limit.child(), FilterExec.class);