PassthroughFile allows writes, but we want to force read-only

[Sebastian Wahl]

The idea is that the mod packages are not allowed to be modified. Configuration files and user data must be manually set up to be stored elsewhere. (Not yet done, it would be great if it could all be configured with another mod.)

So a package which just transparently replicates files should not be allowed to have write access, but this is currently the case. That is not to say that PassthroughFile should not support writing, but it should not be possible to use it for files located in a package.