Rely on rpgp functionality to detect ASCII-armor when reading packets
In !8 (merged) (!8 (comment 316126), !8 (comment 316125), !8 (comment 315697)) we discussed using a (currently not existing) functionality in rPGP in the future to allow reading of buffers (e.g. from split OpenPGP packet files) that automatically deals with binary or ASCII-armored OpenPGP data.
This upstream functionality is tracked in https://github.com/rpgp/rpgp/issues/638