Comment 0 for bug 1957123

The focal and jammy desktop images include a gtk-common-themes snap which uses base: bare. This is obviously supposed to mean the snap has no external dependencies on a particular core snap. Unfortunately, the logic in livecd-rootfs is broken, and when processing the bare snap itself, seeing that it has no base: declared in its own metadata, interprets this as meaning there is an implicit dependency on the core snap and pulls in a whole separate root filesystem pointlessly.

We should:
- fix livecd-rootfs to not look for base snaps of snaps which are themselves type: base
- make it a fatal build error for focal an later to try to build an image that pulls in the core snap, since this is legacy (the Ubuntu 16.04 rootfs should not be used in Ubuntu 20.04 and later images) and it's a bad bug if it does get pulled in.