Fix `no-duplicate-selectors` false negatives for equivalent compound selectors

### What minimal example or steps are needed to reproduce the bug?

```css
.a.b {}
/* … */
.b.a {}
```

```css
#a.b {}
/* … */
.b#a {}
```

```css
div.a.b {}
/* … */
div.b.a {}
```

### What minimal configuration is needed to reproduce the bug?

```json
{
  "rules": {
    "no-duplicate-selectors": true
  }
}
```

ref: https://drafts.csswg.org/selectors/#compound