Let core.get_mod_storage be called multiple times (#12572)

builtin/common/mod_storage.lua

@@ -0,0 +1,19 @@
+-- Modify core.get_mod_storage to return the storage for the current mod.
+
+local get_current_modname = core.get_current_modname
+
+local old_get_mod_storage = core.get_mod_storage
+
+local storages = setmetatable({}, {
+	__mode = "v", -- values are weak references (can be garbage-collected)
+	__index = function(self, modname)
+		local storage = old_get_mod_storage(modname)
+		self[modname] = storage
+		return storage
+	end,
+})
+
+function core.get_mod_storage()
+	local modname = get_current_modname()
+	return modname and storages[modname]
+end