Fails to install core snap if udev is not installed

[Colin Watson]

If udev isn't installed, then snapd fails when trying to install the core snap:

  error: cannot perform the following tasks:
  - Setup snap "core" (3247) security profiles (cannot setup udev for snap "core": cannot reload udev rules: exec: "udevadm": executable file not found in $PATH
  udev output:
  )
  - Setup snap "core" (3247) security profiles (cannot reload udev rules: exec: "udevadm": executable file not found in $PATH
  udev output:
  )

I think snapd should probably depend on udev to avoid this problem.

I appreciate that udev being installed is an uncommon situation, but it isn't in the set of essential packages and so it's possible to have environments without it, particularly ones that don't expect to go through a normal boot process. In particular, I encountered this when working on https://bugs.launchpad.net/launchpad-buildd/+bug/1730376, after fixing the first failure shown there; some buildd base images don't have udev installed. (To work around this, I will probably at least temporarily cause launchpad-buildd to install udev in the target container for builds that need snapd, but it seems to me that the correct fix is to add the dependency.)