[-] BO : fixed temp file getting lost in AdminImportController::split()

This is similar in nature to the fix by @vAugagneur PrestaShop/PrestaShop@8d7196daf3b39d280c631882f0e3ebe473233d07 but instead of retaining the multiple open/close of the file and code that forces a bunch of (slow) disk I/O, it is all done in memory and the file descriptor is only opened and closed once (with no unlinking of the file required).