shiftfs is not available without installing linux-modules-extras package

The shiftfs module, used by lxd, is not available out-of-the-box on cloud images (I've tested on aws, azure and gcp so far). The shiftfs module is installed with the `linux-modules-extra-$(uname -r)` package. This package is not seeded into the cloud images by default. Two potential solutions to this are:

 * Move the shiftfs module to the `linux-modules-$(uname -r)` package.
 * Seed the `linux-modules-extra-$(uname -r)` package into the cloud image.

This all works for the generic kernel. When investigating why, I see that shiftfs.ko is included in linux-modules-5.15.0-78-generic (not the `linux-modules-extra` package). So it appears the location of shiftfs.ko is inconsistent.