### 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 source = as(agg.child(), EsRelation.class);
    }
    public void testPruneMixedAggInsideUnusedEval() {
        var plan = plan("""
              from test
            | stats c = count(salary), max = max(salary), min = min(salary)
            | eval x = max + min + c
            | eval y = min
            | where y > 10
            | eval z = c
            | keep c
            """);
        var project = as(plan, Project.class);
        var limit = as(project.child(), Limit.class);
        var filter = as(limit.child(), Filter.class);
        var eval = as(filter.child(), Eval.class);
        var agg = as(eval.child(), Aggregate.class);
        assertThat(agg.groupings(), hasSize(0));
        var aggs = agg.aggregates();
        assertThat(aggs, hasSize(2));
        assertThat(Expressions.names(aggs), contains("c", "min"));
        var source = as(agg.child(), EsRelation.class);
    }
    public void testNoPruningWhenDealingJustWithEvals() {
        var plan = plan("""
              from test
            | stats c = count(salary), max = max(salary), min = min(salary)
            | eval x = max + min + c
            | eval y = min
            | eval z = c