drop '..' from parts other than subpath

Currently `..` is allowed:

```
>>> import packageurl
>>> packageurl.PackageURL.from_string('pkg:sid/busybox.org/../busybox@1.35.0')
PackageURL(type='sid', namespace='busybox.org/..', name='busybox', version='1.35.0', qualifiers={}, subpath=None)
```

Is this intended behaviour? I would suggest dropping `..`.