Allow Page Manager paths to be imported

Without this patch prepareRow() will ignore valid paths that are not url aliases. The main example of such a path is news-archve which was created by Page Manager (http://330-openberkeley-drops.pantheon.berkeley.edu/admin/structure/pages). 

To test this add 3 rows to openberkeley_starter.menu.csv: 1. path=news-archive 2. path=valid url alias 3. path=invalid-path

David might be the best person to review this PR.
This could be written more simply as:

```
elseif (!drupal_valid_path($row->path)) {
```

But otherwise, the logic looks good! :-)