refactor: Address issues raised by linter

src/ltn12.lua

@@ -13,7 +13,7 @@ local unpack = unpack or table.unpack
 local base = _G
 local _M = {}
 if module then -- heuristic for exporting a global package table
-    ltn12 = _M
+    ltn12 = _M  -- luacheck: ignore
 end
 local filter,source,sink,pump = {},{},{},{}
 
@@ -23,7 +23,6 @@ _M.sink = sink
 _M.pump = pump
 
 local unpack = unpack or table.unpack
-local select = base.select
 
 -- 2048 seems to be better in windows...
 _M.BLOCKSIZE = 2048