Now that the whole lua config is exposed via the extraLuaConfig option,
it's not "extra" anymore but the whole content. Renaming it to "initLua"
is more adequate and it makes the option more understandable I hope.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
