emacsWrapper: preload autoloads

This commits changes the Emacs wrapper, in order to preload all autoload
definitions when built with additional packages. The list of all
definitions is generated at build-time. Packages do not need to
be (require)d for them to work.

Before this change, a code like

```sh
nix-shell -I "nixpkgs=$PWD" -p "emacs.pkgs.withPackages(e:[e.magit])" \
          --run "emacs -Q -nw -f magit"
```

will fail with the message `Symbol’s function definition is void: magit`

After the change, the same code above will open Emacs with magit
enabled.

A slightly longer startup time of ~10ms was detected in local, informal
experiments.

More information on autoloading:
https://www.gnu.org/software/emacs/manual/html_node/eintr/Autoload.html

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