emacs: populate load-path recursively
emacs: populate load-path recursively

Previous behavior only added <nix_profile>/share/emacs/site-lisp/ and
one level of subdirectories, whereas this commit adds all subdirectories
recursively using normal-top-level-add-subdirs-to-load-path. This
matches the behavior of Emacs on non-Nix distros, which include a
/usr/share/emacs/site-lisp/subdirs.el file with identical recursive
behavior.

See also: https://www.emacswiki.org/emacs/LoadPath

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs