bitbake-layers: fix show-appends and flatten for recent cooker changes

Fixes the show-appends and flatten subcommands for recent refactoring in
cooker.

[YOCTO #4536]
[YOCTO #4535]

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>