### 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
*  \_ExchangeExec[REMOTE_SOURCE]
     *    \_ExchangeExec[REMOTE_SINK]
     *      \_ProjectExec[[_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, !gender, languages{f}#5, last_name{f}#6, salary{f}#7]]
     *        \_FieldExtractExec[_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, !ge..]
     *          \_EsQueryExec[test], query[{"bool":{"must_not":[{"term":{"emp_no":{"value":10010}}}],"boost":1.0}}][_doc{f}#9],
     *                                     limit[10000], sort[]
     */
    public void testPushDownNegatedEquality() {
        var plan = physicalPlan("""
            from test
            | where not emp_no == 10010
            """);
        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 source = source(extractRest.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        var boolQuery = as(sv(source.query(), "emp_no"), BoolQueryBuilder.class);
        assertThat(boolQuery.mustNot(), hasSize(1));
        var termQuery = as(boolQuery.mustNot().get(0), TermQueryBuilder.class);
        assertThat(termQuery.fieldName(), is("emp_no"));
        assertThat(termQuery.value(), is(10010));  
    }
    /**
     * Expected:
     *  LimitExec[10000[INTEGER]]
     *  \_ExchangeExec[REMOTE_SOURCE]
     *    \_ExchangeExec[REMOTE_SINK]