[Bug 1957123] [NEW] livecd-rootfs incorrectly installs core snap as implicit dep of other base snaps

[Steve Langasek]

Public bug reported:

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.

** Affects: livecd-rootfs (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1957123

Title:
  livecd-rootfs incorrectly installs core snap as implicit dep of other
  base snaps

Status in livecd-rootfs package in Ubuntu:
  New

Bug description:
  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.

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