fix the xpath for field selection so that fieldset//fields//field is supported once again.

We lost this functionality in Joomla 3 because we want to be able to nest <field> within <field> for the purpose of making repeatable fields. This is a better xpath that avoids selecting <field> tags that are nested within other <field> tags but does select them in any other structure. In other words, this will work again:

```xml
<fieldset>
    <fields>
        <field />
    </fields>
</fieldset>
```