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.

0 files changed, 0 insertions, 0 deletions