### 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
a = as(aggs.get(2), Alias.class);
        assertThat(a.name(), is("s"));
    }
    /**
     * Expected
     * Limit[10000[INTEGER]]
     * \_Aggregate[[last_name{f}#9],[PERCENTILE(salary{f}#10,50[INTEGER]) AS m, last_name{f}#9]]
     *   \_EsRelation[test][_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, !g..]
     */
    public void testMedianReplacement() {
        var plan = plan("""
              from test
            | stats m = median(salary) by last_name
            """);
        var limit = as(plan, Limit.class);
        var agg = as(limit.child(), Aggregate.class);
        assertThat(Expressions.names(agg.aggregates()), contains("m", "last_name"));
        var aggs = agg.aggregates();
        var a = as(aggs.get(0), Alias.class);
        var per = as(a.child(), Percentile.class);
        var literal = as(per.percentile(), Literal.class);
        assertThat((int) QuantileStates.MEDIAN, is(literal.fold()));
        assertThat(Expressions.names(agg.groupings()), contains("last_name"));
    }
    public void testSplittingInWithFoldableValue() {
        FieldAttribute fa = getFieldAttribute("foo");
        In in = new In(EMPTY, ONE, List.of(TWO, THREE, fa, L(null)));
        Or expected = new Or(EMPTY, new In(EMPTY, ONE, List.of(TWO, THREE)), new In(EMPTY, ONE, List.of(fa, L(null))));
        assertThat(new LogicalPlanOptimizer.SplitInWithFoldableValue().rule(in), equalTo(expected));
    }