I've been working for a long time towards automatic nix dependencies for
neovim plugins (using luarocks rockspecs to discover the said
dependencies).
This is an initial commit to help me complete the missing bits.
buildNeovimPluginFrom2Nix is right now a placeholder which helps me test
in my fork a version that does a flat install of luarocks.
the vim updater will now check for attributes with the same name in the lua package set,
if that's the case the script will generate buildNeovimPluginFrom2Nix.
update.py and its companion update-shell.nix need to know where they are, else
they can't find the root default.nix of Nixpkgs. Because of this, I further
added a small piece of documentation about those path-dependent pieces of code.
