R1:98c1267e5f16

Countdown: fix PhutilMissingSymbolException

Summary:
A previous change added a couple of Countdown-related classes:

7ed35123a347a05c70c97eba2bec2b36eb2b3218

This change updates the related library map (arc liberate) in order to
fix any related regression (like PhutilMissingSymbolException).

Closes T15271

Test Plan:
- you run `arc liberate` and the file does not change
- you do not see any `PhutilMissingSymbolException` exception

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1…