Issue #2925899 by quietone, Wim Leers, ravi.shankar, vsujeetkumar,...

Issue #2925899 by quietone, Wim Leers, ravi.shankar, vsujeetkumar, benjifisher, alexpott, mikelutz, larowlan, mpp: MigrateUpgradeImportBatch does not use source_private_file_path & source_base_path correctly, making it impossible to have public & private files in separate locations