manual_add_modules is slow / consumes the most time

I sprinkled mkinitramfs with "date -Ins" to see where most time is spent. I ran the [1] test case in a chroot on my laptop. mkinitramfs took 18.3 seconds. The most time consuming parts:

* 10.13 s (51.4%) auto_add_modules
* 7.20 s (36.5%) run_scripts_optional /usr/share/initramfs-tools/hooks
* 1.27 s (6.4%) final { cat; cpio; cpio|compress } > $outfile

The remaining 1.11 seconds are spread over the remaining parts.

Following hooks in /usr/share/initramfs-tools/hooks took the longest:

* 4.56s (63.3%) framebuffer
* 0.87s (12.1%) plymouth
* 0.81s (11.3%) cryptroot
* 0.23s (3.2%) lvm2
* 0.18s (2.5%) udev
* 0.17s (2.4%) mdadm

The remaining 0.38 seconds are spread over the remaining dozen of hooks.

The framebuffer hook just calls copy_modules_dir and manual_add_modules multiple times. auto_add_modules calls copy_modules_dir multiple times and manual_add_modules. copy_modules_dir calls find and then
manual_add_modules.

So most time will be spent in manual_add_modules. This function calls modprobe on the modules, copies the modules, and add_firmware on the firmwares from a "modinfo -F firmware" call.

Replacing manual_add_modules by a call to /usr/lib/dracut/dracut-install speeds up the initramfs building significantly. In the chroot on my laptop the mkinitramfs execution reduces from 18.3 seconds to 11.0 (40% reduction).

[1] schroot-wrapper -p linux-image-generic,initramfs-tools,zstd,busybox-initramfs,cryptsetup-initramfs,isc-dhcp-client,kbd,lvm2,mdadm,ntfs-3g,plymouth,plymouth-theme-spinner --ppa bdrung/ppa -c mantic -u root