Merge duplicated readdir functionality

internal/archiver.readdir and internal/fs.ReadDir were unused.

internal/fs.ReadDirNames and internal/archiver.readdirnames were doing
nearly the same thing, except one sorted its output and opened with
fs.O_NOFOLLOW. Both were only used in internal/archiver.

internal/fs/fs_helpers.go

@@ -1,45 +0,0 @@
-package fs
-
-import "os"
-
-// ReadDir reads the directory named by dirname within fs and returns a list of
-// directory entries.
-func ReadDir(fs FS, dirname string) ([]os.FileInfo, error) {
-	f, err := fs.Open(dirname)
-	if err != nil {
-		return nil, err
-	}
-
-	entries, err := f.Readdir(-1)
-	if err != nil {
-		return nil, err
-	}
-
-	err = f.Close()
-	if err != nil {
-		return nil, err
-	}
-
-	return entries, nil
-}
-
-// ReadDirNames reads the directory named by dirname within fs and returns a
-// list of entry names.
-func ReadDirNames(fs FS, dirname string) ([]string, error) {
-	f, err := fs.Open(dirname)
-	if err != nil {
-		return nil, err
-	}
-
-	entries, err := f.Readdirnames(-1)
-	if err != nil {
-		return nil, err
-	}
-
-	err = f.Close()
-	if err != nil {
-		return nil, err
-	}
-
-	return entries, nil
-}