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

pkgs/build-support/emacs/wrapper.nix

@@ -135,9 +135,14 @@ runCommand
           ''}
         }
 
+        siteAutoloads="$out/share/emacs/site-lisp/nix-generated-autoload.el"
+        touch $siteAutoloads
+
         # Iterate over the array of inputs (avoiding nix's own interpolation)
         for pkg in "''${requires[@]}"; do
           linkEmacsPackage $pkg
+          find $pkg -name "*-autoloads.el" \
+              -exec echo \(load \"{}\" \'noerror \'nomessage\) \; >> $siteAutoloads
         done
 
         siteStart="$out/share/emacs/site-lisp/site-start.el"
@@ -174,12 +179,12 @@ runCommand
           > "$subdirs"
 
         # Byte-compiling improves start-up time only slightly, but costs nothing.
-        $emacs/bin/emacs --batch -f batch-byte-compile "$siteStart" "$subdirs"
+        $emacs/bin/emacs --batch -f batch-byte-compile "$siteStart" "$subdirs" "$siteAutoloads"
 
         ${optionalString nativeComp ''
           $emacs/bin/emacs --batch \
             --eval "(add-to-list 'native-comp-eln-load-path \"$out/share/emacs/native-lisp/\")" \
-            -f batch-native-compile "$siteStart" "$subdirs"
+            -f batch-native-compile "$siteStart" "$subdirs" "$siteAutoloads"
         ''}
       '';
 

pkgs/build-support/emacs/wrapper.sh

@@ -44,4 +44,4 @@ export emacsWithPackages_siteLisp=@wrapperSiteLisp@
 export EMACSNATIVELOADPATH="${newNativeLoadPath[*]}"
 export emacsWithPackages_siteLispNative=@wrapperSiteLispNative@
 
-exec @prog@ "$@"
+exec @prog@ -l cl-loaddefs -l nix-generated-autoload "$@"