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.
Reintroduced warning on development leftovers in 1.5 libraries.

See #1692