### 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 aggregate = as(limit.child(), Aggregate.class);
        assertThat(aggregate.aggregates(), hasSize(1));
        var alias = as(aggregate.aggregates().get(0), Alias.class);
        var count = as(alias.child(), Count.class);
        var eval = as(aggregate.child(), Eval.class);
        assertThat(eval.fields(), hasSize(1));
        var field = as(eval.fields().get(0), Alias.class);
        assertThat(field.name(), is("x"));
        var source = as(eval.child(), EsRelation.class);
    }
    public void testPruneUnusedAggSimple() {
        var plan = plan("""
              from test
            | stats c = count(salary), max = max(salary), min = min(salary)
            | keep c
            """);
        var limit = as(plan, Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        assertThat(agg.groupings(), hasSize(0));
        assertThat(agg.aggregates(), hasSize(1));
        var aggOne = as(agg.aggregates().get(0), Alias.class);
        assertThat(aggOne.name(), is("c"));
        var count = as(aggOne.child(), Count.class);
        var source = as(agg.child(), EsRelation.class);
    }
    public void testPruneUnusedAggMixedWithEval() {
        var plan = plan("""
              from test
            | stats c = count(salary), max = max(salary), min = min(salary)