### 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 enrich = as(filter.child(), Enrich.class);
        assertTrue(enrich.policyName().resolved());
        assertThat(enrich.policyName().fold(), is(BytesRefs.toBytesRef("languages_idx")));
        var eval = as(enrich.child(), Eval.class);
        as(eval.child(), EsRelation.class);
    }
    /**
     * Expects
     * EsqlProject[[a{r}#3, last_name{f}#9]]
     * \_Eval[[__a_SUM_123{r}#12 / __a_COUNT_150{r}#13 AS a]]
     *   \_Limit[10000[INTEGER]]
     *     \_Aggregate[[last_name{f}#9],[SUM(salary{f}#10) AS __a_SUM_123, COUNT(salary{f}#10) AS __a_COUNT_150, last_nam
     * e{f}#9]]
     *       \_EsRelation[test][_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, !g..]
     */
    public void testSimpleAvgReplacement() {
        var plan = plan("""
              from test
            | stats a = avg(salary) by last_name
            """);
        var project = as(plan, Project.class);
        assertThat(Expressions.names(project.projections()), contains("a", "last_name"));
        var eval = as(project.child(), Eval.class);
        var f = eval.fields();
        assertThat(f, hasSize(1));
        assertThat(f.get(0).name(), is("a"));
        var limit = as(eval.child(), Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        var aggs = agg.aggregates();
        var a = as(aggs.get(0), Alias.class);