### 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 testReplaceFilterWithExact() {
        var plan = plan("""
              from test
            | where job == "foo"
            """);
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        Equals equals = as(filter.condition(), Equals.class);
        FieldAttribute left = as(equals.left(), FieldAttribute.class);
        assertThat(left.name(), equalTo("job"));
    }
    public void testReplaceExpressionWithExact() {
        var plan = plan("""
              from test
            | eval x = job
            """);
        var eval = as(plan, Eval.class);
        var alias = as(eval.fields().get(0), Alias.class);
        var field = as(alias.child(), FieldAttribute.class);
        assertThat(field.name(), equalTo("job"));
    }
    public void testReplaceSortWithExact() {
        var plan = plan("""
              from test
            | sort job
            """);
        var topN = as(plan, TopN.class);
        assertThat(topN.order().size(), equalTo(1));
        var sortField = as(topN.order().get(0).child(), FieldAttribute.class);
        assertThat(sortField.name(), equalTo("job"));