### 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/LocalPhysicalPlanOptimizerTests.java
var agg = as(limit.child(), AggregateExec.class);
        assertThat(agg.getMode(), is(FINAL));
        assertThat(Expressions.names(agg.aggregates()), contains("c", "call", "c_literal"));
        var exchange = as(agg.child(), ExchangeExec.class);
        var esStatsQuery = as(exchange.child(), EsStatsQueryExec.class);
        assertThat(esStatsQuery.limit(), is(nullValue()));
        assertThat(Expressions.names(esStatsQuery.output()), contains("count", "seen"));
        var expected = wrapWithSingleQuery(QueryBuilders.rangeQuery("emp_no").gt(10010), "emp_no");
        assertThat(expected.toString(), is(esStatsQuery.query().toString()));
    }
    
    public void testCountFieldsAndAllWithFilter() {
        var plan = plan("""
            from test
            | where emp_no > 10010
            | stats c = count(), cs = count(salary), ce = count(emp_no)
            """, IS_SV_STATS);
        assertThat(plan.anyMatch(EsQueryExec.class::isInstance), is(true));
    }
    private QueryBuilder wrapWithSingleQuery(QueryBuilder inner, String fieldName) {
        return FilterTests.singleValueQuery(inner, fieldName);
    }
    private Stat queryStatsFor(PhysicalPlan plan) {
        var limit = as(plan, LimitExec.class);
        var agg = as(limit.child(), AggregateExec.class);
        var exg = as(agg.child(), ExchangeExec.class);
        var statSource = as(exg.child(), EsStatsQueryExec.class);
        var stats = statSource.stats();
        assertThat(stats, hasSize(1));
        var stat = stats.get(0);