Changed stage-snaps property does not make pull step dirty

[林博仁(Buo-ren, Lin)]

Bug reproduction instructions:

  1. `git clone --branch=bug-stage-snaps-dirtiness https://github.com/Lin-Buo-Ren/snapcraft-bug-poc.git && cd snapcraft-bug-poc`
  2. Run `snapcraft stage ccache`
  3. Uncomment the commented lines in snap/snapcraft.yaml
  4. Run `snapcraft stage ccache` again

  Expected behavior:

      Snapcraft considers ccache part's `pull` step changed and redo the pull step

  Current behavior:

      Snapcraft only considered ccache part's `stage` step changed, then crashed due to the stage snap isn't pull and build yet.