### 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/PhysicalPlanOptimizerTests.java
public void testEnrich() {
        var plan = physicalPlan("""
            from test
            | enrich foo on first_name
            """);
        var optimized = optimizedPlan(plan);
        var enrich = as(optimized, EnrichExec.class);
        var limit = as(enrich.child(), LimitExec.class);
        var exchange = asRemoteExchange(limit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        var source = source(extract.child());
        
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES * 2 + Long.BYTES));
    }
    /**
     * Expects the filter to transform the source into a local relationship
     * LimitExec[10000[INTEGER]]
     * \_ExchangeExec[[],false]
     *   \_LocalSourceExec[[_meta_field{f}#8, emp_no{r}#2, first_name{f}#3, gender{f}#4, languages{f}#5, last_name{f}#6, salary{f}#7],EMPT
     * Y]
     */
    public void testLocallyMissingField() {
        var testStats = statsForMissingField("emp_no");
        var optimized = optimizedPlan(physicalPlan("""
              from test
            | where emp_no > 10
            """), testStats);
        var limit = as(optimized, LimitExec.class);
        var exchange = as(limit.child(), ExchangeExec.class);