### 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 eval = as(plan, Eval.class);
        var limit = as(eval.child(), Limit.class);
        var agg = as(limit.child(), Aggregate.class);
    }
    public void testNoPruningWhenChainedEvals() {
        var plan = plan("""
              from test
            | eval x = emp_no, y = x + 1, z = y
            | keep z
            """);
        var project = as(plan, Project.class);
        var eval = as(project.child(), Eval.class);
        assertThat(Expressions.names(eval.fields()), contains("x", "y", "z"));
        var limit = as(eval.child(), Limit.class);
        var source = as(limit.child(), EsRelation.class);
    }
    public void testPruningDuplicateEvals() {
        var plan = plan("""
              from test
            | eval x = emp_no, x = salary
            | eval y = salary
            | eval y = emp_no
            | 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);
        var source = as(limit.child(), EsRelation.class);