move away from using by-partuuid, and instead find partition block device that can be mounted directly

[Dimitri John Ledkov]

move away from using by-partuuid, and instead fine partition block device that can be mounted directly.

For example, instead of using disk.FindMatchingPartitionUUID("ubuntu-save")=UUID and then appending that to /dev/disky/by-partuuid/UUID and then mounting it.

We could add an api disk.FindMatchingPartition("ubuntu-save") which directly returns /dev/nvme0n1p3 which can then be directly passed to doSystemdMount. Doing that will save on string manipulation; can be done entirely with sysfs, without udevd/udevadm calls, and mocked with files.

Some of this is started in https://github.com/snapcore/snapd/pull/9594/files

[Ian Johnson (anonymouse67)]

All of the primitives are there for this in the disks package, just needs to have the functions changed in the call sites in cmd_initramfs_mounts.go