dev/translation#30 Move l10n resource dir under private files

dev/translation#30 Move l10n resource dir under private files

Looks under [civicrm.private]/l10n if that exists, falling back to
[civicrm.root]/l10n.

Since I18n::getResourceDirectory needs to be called from GenCode,
where we don't have the base directory defined, we make the l10n
path factory function consult new #define CIVICRM_L10N_BASEDIR first.

In GenCode itself, keep using the old location for now, but make
it possible to change the location with an env var as soon as we
update buildkit.