### 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/LogicalPlanOptimizerTests.java
""");
        var limit = as(plan, Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        assertThat(Expressions.names(agg.aggregates()), contains("count(salary)", "x"));
        assertThat(Expressions.names(agg.groupings()), contains("x"));
        var alias = as(agg.aggregates().get(1), Alias.class);
        var field = as(alias.child(), FieldAttribute.class);
        assertThat(field.name(), is("first_name"));
        var group = as(agg.groupings().get(0), Attribute.class);
        assertThat(group, is(alias.toAttribute()));
        var from = as(agg.child(), EsRelation.class);
    }
    /**
     * Expects
     * EsqlProject[[x{r}#3, y{r}#6]]
     * \_Eval[[emp_no{f}#9 + 2[INTEGER] AS x, salary{f}#14 + 3[INTEGER] AS y]]
     *   \_Limit[10000[INTEGER]]
     *     \_EsRelation[test][_meta_field{f}#15, emp_no{f}#9, first_name{f}#10, g..]
     */
    public void testCombineEvals() {
        var plan = plan("""
            from test
            | eval x = emp_no + 2
            | eval y = salary + 3
            | keep x, y
            """);
        var project = as(plan, Project.class);
        var eval = as(project.child(), Eval.class);
        assertThat(Expressions.names(eval.fields()), contains("x", "y"));
        var limit = as(eval.child(), Limit.class);