emacs: resolve wrapper load-path at build time

Since the included package set is determined at build time we can also
generate the `subdirs.el` file at build time. This improves startup
time somewhat since we don't have to traverse the directory to add to
`load-path`.

For example,

``` sh-session
$ bench './emacs-old -Q --batch --kill' './emacs-new -Q --batch --kill'
benchmarking bench/./emacs-old -Q --batch --kill
time 72.77 ms (71.66 ms .. 73.65 ms)
1.000 R² (0.999 R² .. 1.000 R²)
mean 72.49 ms (72.06 ms .. 72.92 ms)
std dev 746.5 μs (582.4 μs .. 1.008 ms)

benchmarking bench/./emacs-new -Q --batch --kill
time 40.56 ms (40.24 ms .. 40.86 ms)
1.000 R² (0.999 R² .. 1.000 R²)
mean 40.30 ms (40.12 ms .. 40.51 ms)
std dev 401.9 μs (311.1 μs .. 555.8 μs)
```

The change does not actually affect the content of `load-path`:

``` sh-session
$ diff -s <(./emacs-old --batch --eval '(prin1 load-path)' | sed -E 's!/nix/store/[[:alnum:]]{32}-!!g') \
<(./emacs-new --batch --eval '(prin1 load-path)' | sed -E 's!/nix/store/[[:alnum:]]{32}-!!g')
Files /dev/fd/63 and /dev/fd/62 are identical
```

So in principle the only observable effect should be the improved
startup time.

pkgs/build-support/emacs/mk-wrapper-subdirs.el

···
+(defmacro mk-subdirs-expr (path)
+  `(setq load-path
+         (delete-dups (append '(,path)
+                              ',(let ((default-directory path))
+                                  (normal-top-level-add-subdirs-to-load-path))
+                              load-path))))

pkgs/build-support/emacs/wrapper.nix

···
           (add-to-list 'native-comp-eln-load-path "$out/share/emacs/native-lisp/")
         ''}
         EOF
-        # Link subdirs.el from the emacs distribution
-        ln -s $emacs/share/emacs/site-lisp/subdirs.el -T $subdirs
+
+        # Generate a subdirs.el that statically adds all subdirectories to load-path.
+        $emacs/bin/emacs \
+          --batch \
+          --load ${./mk-wrapper-subdirs.el} \
+          --eval "(prin1 (macroexpand-1 '(mk-subdirs-expr \"$out/share/emacs/site-lisp\")))" \
+          > "$subdirs"
 
         # Byte-compiling improves start-up time only slightly, but costs nothing.
         $emacs/bin/emacs --batch -f batch-byte-compile "$siteStart" "$subdirs"