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.
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