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.
We change the current logic: instead of writing an init.vim which loads
lua/init-home-manager.lua, we write an init.lua that sources init.vim
This commit also avoids writing any of these files if the plugins have
no config.
