FIX: _config/ directories are now correctly detected as modules (fixes #1762)

DO NOT MERGE: to be reviewed. Only i18n & Deprecation classes use
->getModules() as far as I can see. Given that the method still simply
returns an array of modulename => modulepath, I don't think it's really
an API change
Merge pull request #1770 from kinglozzer/config-dir-detection-fix

FIX: _config/ directories are now correctly detected as modules (fixes #1762)