now that home-manager always generates an init.lua we can take the
wrapper output instead of reimplemnting stuff in HM (notwithstanding rtp
changes that are not needed in HM).
This is necessary to keep the provider configuration available after
this change https://github.com/NixOS/nixpkgs/pull/487390
Also updated test to reflect ruby disabling
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.
