emacs: fix elisp native compilation errors caused by load-path

If $pname-$version/foo.el requires $pname-$version/bar-dir/bar.el,
previously there were native compilation errors like the below one
because $pname-$version/bar-dir was not added to load-path (only
$pname-$version is added).

Error: file-missing ("/nix/store/hash-emacs-sly-20240809.2119/share/emacs/site-lisp/elpa/sly-20240809.2119/contrib/sly-mrepl.el" "Cannot open load file" "No such file or directory" "sly-autodoc")

Currently, these packages are affected: haskell-tng-mode[^1],
psgml[3], sly[4], ess[5], el-get[6], proof-general[7], hyperbole[8]
and edts[9].

At run time, we recursively[1] add[2] $pname-$version and its subdirs
to load-path.  Let's also do that at build time to fix this kind of
errors.

[1]: https://github.com/NixOS/nixpkgs/blob/9625766c32942d4fdeef6cfee0d4337f75697b5f/pkgs/applications/editors/emacs/site-start.el#L18
[2]: https://github.com/NixOS/nixpkgs/blob/9625766c32942d4fdeef6cfee0d4337f75697b5f/pkgs/applications/editors/emacs/build-support/wrapper.nix#L170-L175
[3]: https://hydra.nixos.org/build/271291118/nixlog/1
[4]: https://hydra.nixos.org/build/271372072/nixlog/1
[5]: https://hydra.nixos.org/build/271284390/nixlog/1
[6]: https://hydra.nixos.org/build/271385904/nixlog/1
[7]: https://hydra.nixos.org/build/271277707/nixlog/1
[8]: https://hydra.nixos.org/build/271345526/nixlog/1
[9]: https://hydra.nixos.org/build/271290054/nixlog/1
[^1]: Errors of this kind are shadowed by other errors.

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