### 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 plan = physicalPlan("""
            from test
            | where not first_name rlike ".*foo.*"
            """);
        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));
        QueryBuilder query = source.query();
        assertNotNull(query);
        assertThat(query, instanceOf(BoolQueryBuilder.class));
        var boolQuery = (BoolQueryBuilder) query;
        List<QueryBuilder> mustNot = boolQuery.mustNot();
        assertThat(mustNot.size(), is(1));
        assertThat(mustNot.get(0), instanceOf(RegexpQueryBuilder.class));
        var regexpQuery = (RegexpQueryBuilder) mustNot.get(0);
        assertThat(regexpQuery.fieldName(), is("first_name"));
        assertThat(regexpQuery.value(), is(".*foo.*"));
    }
    /**
     * EnrichExec[first_name{f}#3,foo,fld,idx,[a{r}#11, b{r}#12]]
     *  \_LimitExec[10000[INTEGER]]
     *    \_ExchangeExec[]
     *      \_ProjectExec[[_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, gender{f}#4, languages{f}#5, last_name{f}#6, salary{f}#7]]
     *        \_FieldExtractExec[_meta_field{f}#8, emp_no{f}#2, first_name{f}#3, gen..]
     *          \_EsQueryExec[test], query[][_doc{f}#13], limit[10000], sort[] estimatedRowSize[216]
     */