fix(internal/fsx): remove pre Go 1.16 definitions (#270)

Occurred to me while working on https://github.com/ooni/probe/issues/1299.

internal/engine/internal/fsx/fsx.go

@@ -3,31 +3,18 @@ package fsx
 
 import (
 	"fmt"
+	"io/fs"
 	"os"
 	"syscall"
 )
 
-// File is a generic file. This interface is taken from the draft
-// iofs golang design. We'll use fs.File when available.
-type File interface {
-	Stat() (os.FileInfo, error)
-	Read([]byte) (int, error)
-	Close() error
-}
-
-// FS is a generic file system. Like File, it's adapted from
-// the draft iofs golang design document.
-type FS interface {
-	Open(name string) (File, error)
-}
-
 // Open is a wrapper for os.Open that ensures that we're opening a file.
-func Open(pathname string) (File, error) {
+func Open(pathname string) (fs.File, error) {
 	return OpenWithFS(filesystem{}, pathname)
 }
 
 // OpenWithFS is like Open but with explicit file system argument.
-func OpenWithFS(fs FS, pathname string) (File, error) {
+func OpenWithFS(fs fs.FS, pathname string) (fs.File, error) {
 	file, err := fs.Open(pathname)
 	if err != nil {
 		return nil, err
@@ -39,13 +26,16 @@ func OpenWithFS(fs FS, pathname string) (File, error) {
 	}
 	if info.IsDir() {
 		file.Close()
-		return nil, fmt.Errorf("input path points to a directory: %w", syscall.EISDIR)
+		return nil, fmt.Errorf(
+			"input path points to a directory: %w", syscall.EISDIR)
 	}
 	return file, nil
 }
 
+// filesystem is a private implementation of fs.FS.
 type filesystem struct{}
 
-func (filesystem) Open(pathname string) (File, error) {
+// Open implements fs.FS.Open.
+func (filesystem) Open(pathname string) (fs.File, error) {
 	return os.Open(pathname)
 }

internal/engine/internal/fsx/fsx_test.go

@@ -2,6 +2,7 @@ package fsx_test
 
 import (
 	"errors"
+	"io/fs"
 	"os"
 	"sync/atomic"
 	"syscall"
@@ -26,8 +27,8 @@ func (FailingStatFile) Stat() (os.FileInfo, error) {
 	return nil, errStatFailed
 }
 
-func (fs FailingStatFS) Open(pathname string) (fsx.File, error) {
-	return FailingStatFile{CloseCount: fs.CloseCount}, nil
+func (f FailingStatFS) Open(pathname string) (fs.File, error) {
+	return FailingStatFile(f), nil
 }
 
 func (fs FailingStatFile) Close() error {