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.