make AbstractPlugin::pi_loadLL load labels from custom file
Right now AbstractPlugin::pi_loadLL looks for hardcoded
language.xml file in the scirpt directory "scriptRelPath".
'EXT:' . $this->extKey . '/' . dirname($this->scriptRelPath) . '/locallang.xlf';
This doesn't allow to use Extbase like structure for pi-based extensions (placing language lables in Resources/Private/Language
In few places in the core, we still keep scriptRelPath containing wrong value (script location and name has already changed during namespace change). And the only reason for it is that we need this property for loading language files.
The solution would be to make AbstractPlugin::pi_loadLL take an optional argument pointing to the correct loaction of the language file.