[BUGFIX] Externally-referenced ll-XML files are not parsed

It is not possible to load an externally-referenced localization file
as explained in the documentation:

<languageKey index="fr">EXT:yourext/path/to/fr.locallang.xml</languageKey>

Change-Id: I2461a77c98779165bf6a39b0d73fdda4984a1399
Fixes: #29313
Releases: 4.6
Reviewed-on: http://review.typo3.org/4651
Reviewed-by: Dominique Feyer
Tested-by: Dominique Feyer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers