### 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
mapping = loadMapping("mapping-basic.json");
        allFieldRowSize = mapping.values()
            .stream()
            .mapToInt(
                f -> (EstimatesRowSize.estimateSize(EsqlDataTypes.widenSmallNumericTypes(f.getDataType())) + f.getProperties()
                    .values()
                    .stream()
                    
                    .mapToInt(x -> EstimatesRowSize.estimateSize(EsqlDataTypes.widenSmallNumericTypes(x.getDataType())))
                    .sum())
            )
            .sum();
        EsIndex test = new EsIndex("test", mapping);
        IndexResolution getIndexResult = IndexResolution.valid(test);
        logicalOptimizer = new LogicalPlanOptimizer();
        physicalPlanOptimizer = new PhysicalPlanOptimizer(new PhysicalOptimizerContext(config));
        FunctionRegistry functionRegistry = new EsqlFunctionRegistry();
        mapper = new Mapper(functionRegistry);
        var enrichResolution = new EnrichResolution(
            Set.of(
                new EnrichPolicyResolution(
                    "foo",
                    new EnrichPolicy(EnrichPolicy.MATCH_TYPE, null, List.of("idx"), "fld", List.of("a", "b")),
                    IndexResolution.valid(
                        new EsIndex(
                            "idx",
                            Map.ofEntries(
                                Map.entry("a", new EsField("a", DataTypes.INTEGER, Map.of(), true)),
                                Map.entry("b", new EsField("b", DataTypes.LONG, Map.of(), true))
                            )