The package installer now asks you if you want to move the installer to the trash after installation (it's been this way for a few OS releases now). This will

The package installer now asks you if you want to move the installer to the trash after installation (it's been this way for a few OS releases now). This will eject the .dmg and delete it for you as well. I think it only does this if the .dmg is in the Downloads directory though.