core: move expansion checks into module header

mods/target-debufftimer.lua

@@ -1,17 +1,14 @@
 local _G = _G or getfenv(0)
-local libdebuff = ShaguTweaks.libdebuff
-local GetExpansion = ShaguTweaks.GetExpansion
-
--- there are already debuff timers in tbc+
-if GetExpansion() ~= "vanilla" then return end
 
 local module = ShaguTweaks:register({
   title = "Debuff Timer",
   description = "Show debuff durations on the target unit frame.",
+  expansions = { ["vanilla"] = true, ["tbc"] = nil },
   enabled = true,
 })
 
-local UnitDebuff = ShaguTweaks.libdebuff.UnitDebuff
+local libdebuff = ShaguTweaks.libdebuff
+local UnitDebuff = libdebuff and libdebuff.UnitDebuff
 
 local function TimeConvert(remaining)
   local color = "|cffffffff"