### 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
TEST_VERIFIER
        );
    }
    public void testEmptyProjections() {
        var plan = plan("""
            from test
            | keep salary
            | drop salary
            """);
        var relation = as(plan, LocalRelation.class);
        assertThat(relation.output(), is(empty()));
        assertThat(relation.supplier().get(), emptyArray());
    }
    public void testEmptyProjectionInStat() {
        var plan = plan("""
            from test
            | stats c = count(salary)
            | drop c
            """);
        var relation = as(plan, LocalRelation.class);
        assertThat(relation.output(), is(empty()));
        assertThat(relation.supplier().get(), emptyArray());
    }
    public void testCombineProjections() {
        var plan = plan("""
            from test
            | keep emp_no, *name, salary
            | keep last_name
            """);
        var keep = as(plan, Project.class);