Don't forbid unknown files in a library

The current code forbids any files it does not know about, but this is
bad because:
 - It breaks forward compatibility if we later add more files or
   directories to the library format.
 - It breaks for people who want to have some extra stuff in their
   library (say, .gitignore or a README file). We can't keep a list of
   "allowed" stuff, since there will always be stuff missing.

This commit removes that code and just allows all files again.