fix(build): distinguish vim.mpack from global require'mpack'

problem: the api of vim.mpack is not compatible with a system provided mpack
solution: don't require 'mpack' directly from the system path

src/nlua0.c

@@ -0,0 +1,13 @@
+#include <lua.h>
+
+#include "mpack/lmpack.h"
+
+LUA_API int luaopen_nlua0(lua_State* L);
+
+LUA_API int luaopen_nlua0(lua_State* L) {
+  lua_getglobal(L, "vim");
+  luaopen_mpack(L);
+  lua_setfield(L, -2, "mpack");
+
+  return 1;
+}