A PKG file is essentially an encrypted archive containing EBOOT.BIN (the executable), assets, sounds, videos, and metadata. When installed via the PS3's XMB (XrossMediaBar), the system decrypts and extracts the contents to the internal hard drive.
A major draw for revisiting MW2 is the multiplayer experience. However, official PlayStation Network (PSN) servers for the game were not shut down, but they are riddled with hackers and dead lobbies. Using a PKG installation, you have two paths: Call Of Duty Modern Warfare 2 Ps3 Pkg