include dmsetup in core snaps

[Justification]
The dmsetup package includes udev rules that are essential for any system using device mapper, which includes any system using crypsetup; and we want to support full-disk encryption in Ubuntu Core.

[Test case]
Do a livefs build test with PROJECT=ubuntu-core SUBPROJECT=system-image EXTRA_PPAS='snappy-dev/image snappy-dev/edge' IMAGEFORMAT=plain to confirm that the build succeeds. All other CI will be handled downstream of the livecd-rootfs SRU as part of the snap build and publication process.

[Regression potential]
This adds a further package into the core and core16 snaps, which will increase the size. The size increase is only marginal (~230KiB uncompressed), but may still adversely impact some installs by pushing the snap size over some limit.

This also changes the boot-time behavior of udev by introducing new udev rules. These rules are well-tested and should only have effect on a system which is configured to create device-mapper devices, which is not going to be the case for a stock Ubuntu Core system; however, some risk of regression exists.