Import game controller code on first use

[pvcraven]

Closes #1159. Related to #2402: this PR covers import time only, not which names should be top-level.

Why import arcade was slow

Measured with python -X importtime on Windows:

Import	Time	Cause
pyglet.input (via arcade.joysticks)	1.35 s	pyglet creates DirectInput and lists the connected devices when imported
pyglet.media (via arcade.sound)	0.50 s	pyglet opens the audio driver when imported
everything else	about 0.4 s	window, graphics and text

Change

• get_controllers, ControllerManager, get_joysticks, get_game_controllers, and the arcade.controller / arcade.joysticks modules are loaded on first use, through a module __getattr__.
  • Type checkers still see them through TYPE_CHECKING imports.
  • from arcade import ... and from arcade import * work as before.
  • In headless mode they're still missing, as before.
• Loading Arcade's extra controller mappings (gamecontrollerdb.txt) moved from arcade/__init__.py to arcade/controller.py.
• arcade/input/manager.py, arcade/gui/ui_manager.py and arcade/experimental/controller_window.py only used pyglet's Controller for type hints. They now import it for type checkers only. ControllerWindow imports ControllerManager when it's created.
• The experimental input_manager_example uses arcade.ControllerManager() instead of pyglet's, so it keeps Arcade's mappings.

Sound is unchanged. It still loads at import. Making it lazy would cut another 0.5 s, but then the first load_sound could make the game stutter if it happens mid-game. A pause when connecting controllers is fine, because that normally happens at startup.

Results (Windows)

	Before	After
import arcade	about 2.7 s	about 0.9 s
first load_sound / play()	instant	instant (audio driver opened at import)
first get_controllers()	(paid at import)	about 1.5 s

Behavior change

Code that calls pyglet.input.get_controllers() directly, without using Arcade's controller functions first, now gets pyglet's 302 built-in mappings instead of Arcade's 2147. The changelog says so.

Testing

• The full test suite passes (1843), and ruff, mypy, pyright and the docs build are clean.
• The sound, controller and InputManager examples, and the GUI controller examples (ControllerWindow), run without errors.
• Headless mode can't run on Windows, so CI's Linux job covers it.

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