fs: rewrite Reader to build fs tree up front

This adds proper support for filenames that include directories. For
example, `/foo/bar` would result in an error when trying to open `/foo`.

The directory tree is now build upfront. This ensures let's the
directory tree construction be handled only once. All accessors then
only have to look up the constructed directory entries.

cmd/restic/cmd_backup.go

@@ -591,12 +591,10 @@ func runBackup(ctx context.Context, opts BackupOptions, gopts GlobalOptions, ter
 				return err
 			}
 		}
-		targetFS = &fs.Reader{
-			ModTime:    timeStamp,
-			Name:       filename,
-			Mode:       0644,
-			ReadCloser: source,
-		}
+		targetFS = fs.NewReader(filename, source, fs.ReaderOptions{
+			ModTime: timeStamp,
+			Mode:    0644,
+		})
 		targets = []string{filename}
 	}