Take over from ShaguTweaks when both are installed

ShagaTweaks and the original ShaguTweaks share the ShaguTweaks namespace,
so running both applies every tweak twice. When the original is loaded too,
ShagaTweaks now stays idle for that session and turns ShaguTweaks and
ShaguTweaks-extras off, so from the next login or /reload only ShagaTweaks
runs. Settings carry over: both addons save ShaguTweaks_config and
ShaguTweaks_cache, so the session that has both writes the original's
settings into ShagaTweaks' saved variables.

main.lua

@@ -41,6 +41,15 @@ ShaguTweaks:SetScript("OnEvent", function()
     return
   end
 
+  -- ShagaTweaks replaces ShaguTweaks: when the original is loaded as well, it
+  -- runs this session and is turned off from the next one. Its settings carry
+  -- over, as both addons save ShaguTweaks_config and ShaguTweaks_cache.
+  if IsAddOnLoaded("ShaguTweaks") then
+    DisableAddOn("ShaguTweaks")
+    DisableAddOn("ShaguTweaks-extras")
+    return
+  end
+
   -- load current expansion
   local expansion = ShaguTweaks:GetExpansion()