The actual implementation still relies on file paths, but with the
abstraction layer in place, an FS implementation can ensure atomic file
accesses in the future.
The temp files used by the packer manager are either delete after
creation (unix) or marked as delete on close (windows). Thus, no
explicit cleanup is necessary.
Add new generic_attributes attribute in Node.
Use the generic attributes to add support for creation time and file attributes like hidden, readonly, encrypted in windows. Handle permission errors for readonly files in windows.
Handle backup and restore of encrypted attributes using windows system calls.
This adds two implementations of the new `FS` interface: One for the local
file system (`Local`) and one for a single file read from an
`io.Reader` (`Reader`).
