### 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
}
    public void testPruneUnusedEval() {
        var plan = plan("""
              from test
            | eval garbage = salary + 3
            | keep salary
            """);
        var keep = as(plan, Project.class);
        var limit = as(keep.child(), Limit.class);
        var source = as(limit.child(), EsRelation.class);
    }
    public void testPruneChainedEval() {
        var plan = plan("""
              from test
            | eval garbage_a = salary + 3
            | eval garbage_b = emp_no / garbage_a, garbage_c = garbage_a
            | eval garbage_x = 1 - garbage_b/garbage_c
            | keep salary
            """);
        var keep = as(plan, Project.class);
        var limit = as(keep.child(), Limit.class);
        var source = as(limit.child(), EsRelation.class);
    }
    public void testPruneEvalDueToStats() {
        var plan = plan("""
              from test
            | eval garbage_a = salary + 3, x = salary
            | eval garbage_b = x + 3
            | stats c = count(x)
            """);