Rename plugins/ to integration/ to match the cross-project convention

ChestShop's plugins/ package holds exactly the softdepend integrations (Dependencies loader, ItemBridge/Nexo custom-item facades, ShowItemHook, WorldGuard*/GriefPrevention*/VanillaShopProtection). Business names the same concept integration/. Rename plugins/ → integration/ for cross-project consistency.

Pure package move + import/FQN updates + redundant-import cleanup. Runtime-verified (Dependencies is registered as a listener; the Nexo facade is exercised by the integration suite): server enables cleanly, ChestShop suite 42/42.