Allow setting a custom L10n/I18n resource directory

CiviCRM Core contains some code which gets the resource directory for i18n files - at the moment this is set as a "static variable" within the function here.

It would be beneficial if we could set this properly to a directory of our choosing and would allow proper I18n support on Drupal 8 without having to place the L10n folder inside /vendor/civicrm/civicrm-core which complicates composer updates, completely breaks with composer best practices and adds an additional manual step to get going with CiviCRM and Drupal 8.

Allowing a custom setting for this would also allow us to set a sane default ([civicrm.files]/l10n)? which we can then use to "composerify" the inclusion of the L10n tarball (e.g: https://github.com/drupal-composer/drupal-l10n)