Motivation for this change

We need both the .so as well as the proper autoloaders that are generated when
we build an emacs package, so we cannot use the normal stdenv.mkDerivation but
have to inject the building of the libraries instead.

Also, use the proper libvterm-neovim we have in nixpkgs instead of vendoring it.

Without this, spacemacs doesn't detect the package as installed and instead tries to install it itself. I haven't tried with vanilla emacs but without the vterm-pkg.el and auto loader, I don't think it will work.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @lovek323 @peti @the-kenny @jwiegley