Further abstraction for OS interface

doublestar only needs Readdir and the Closer interface for the type returned by OS.Open(). By changing the return type to this basic interface, it's even easier to abstract the calls to the filesystem for testing. This is an API change, though, necessitating a major version bump. Updating should be fairly easy. See: https://github.com/bmatcuk/doublestar/blob/master/UPGRADING.md

Thanks for the suggestion @c4milo!