remasterpup2: *.files -> builtin_files

While remastering, all user-installed-packages go to woof-installed-packages, but corresponding *.files get deleted instead of being copied to .packages/builtin_files/, hence in remastered Pup it's impossible to really uninstall them via remove_builtin utility, despite they're listed there.
Well, the format of stock files in .packages/builtin_files/* differs from .packages/*.files, but remove_builtin is processing both types properly, so no need to convert.