PKG is an archive format used to store digital versions of games on the Playstation Network. The file contents of a PKG are encrypted, but for PSP & PS3 the decryption keys are known.
As of 2017 there is no way to decrypt PS4 or Vita PKGs as the decryption keys for said platforms are not known.
Only the PS3 version of Persona 5 is decryptable at the moment, and therefore possible to mod and datamine.
While the decryption keys for Vita PKGs are not known, there are several methods to dump PSVita games by mounting the game while the system decrypts it as a way around the PKG encryption.
PKGView can extract all the files in a decrypted PKG. Just open the downloaded PKG file, right click the topmost directory and extract to source folder.