Package contents were being extracted into subdirectories of the boot
tree (grub_dir, shim_dir), which meant the boot tree contained both
the final boot files and the raw package extractions. Extract packages
into scratch directories instead, copying only the needed files into
the boot tree. This also removes the grub_dir/shim_dir instance
variables and the create_dirs overrides, and moves copy_grub_modules
to a standalone function in grub.py.
ff3addb2f8