Misleading error message about lxd.migrate

[Alberto Mardegan]

Running `snapcraft --use-lxd` on my Ubuntu 20.04 system led to this error:

    The 'LXD' provider does not support having the 'lxd' or 'lxd-client' deb packages installed. To completely migrate to the LXD snap run 'lxd.migrate' and try again.

Even though I knew my lxd was already coming from the snap version, I ran `lxd.migrate` anyway, and it failed:

    => Connecting to source server
    error: Unable to connect to the source LXD: Get "http://unix.socket/1.0": dial unix /var/lib/lxd/unix.socket: connect: no such file or directory

Running out of ideas, I removed the transitional deb package for lxd, and that indeed solved the issue. But maybe snapcraft should either make the message more clear, or the lxd.migrate tool (or even snapcraft itself) should be made smarter and just check if the lxd snap is installed, without caring about the deb version.

[Chris Patterson (cjp256)]

We will remove the deb check to avoid this unnecessary message.