fs: Add interface and FS implementations

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`).

internal/fs/file.go

@@ -1,25 +1,11 @@
 package fs
 
 import (
-	"io"
 	"os"
 	"path/filepath"
 	"time"
 )
 
-// File is an open file on a file system.
-type File interface {
-	io.Reader
-	io.Writer
-	io.Closer
-
-	Fd() uintptr
-	Readdirnames(n int) ([]string, error)
-	Readdir(int) ([]os.FileInfo, error)
-	Seek(int64, int) (int64, error)
-	Stat() (os.FileInfo, error)
-}
-
 // Mkdir creates a new directory with the specified name and permission bits.
 // If there is an error, it will be of type *PathError.
 func Mkdir(name string, perm os.FileMode) error {