[twig] fix merge of the paths defined in several config files.

|Q	|A
------|-----
|Bug fix?|	yes
|New feature?|	no
|BC breaks?|	no
|Deprecations?|	no
|Tests pass?| yes
|Fixed tickets|	-
|License|	MIT
|Doc PR| -

Let's say I have two separate configs files in which I define `twig.paths`, or an extension that implements `PrependExtensionInterface` and prepend twig paths there. If we have only entry where the `paths` is defined everything works fine. But if there two or more we may ended with php fatal error:

```
Fatal error: Uncaught exception 'Twig_Error_Loader' with message 'The "0" directory does not exist.' in /home/vagrant/projects/Foo/vendor/twig/twig/lib/Twig/Loader/Filesystem.php on line 93
```