vcontainer-common: support nested OCI layout and fix vimport shell errors

The multi-arch OCI functions (is_oci_image_index, get_oci_platforms,
select_platform_manifest) only checked index.json directly for platform
information. With the skopeo-compatible nested OCI layout — where
index.json references a single image index blob that in turn contains
the per-platform manifests — the functions failed to detect multi-arch
images because index.json no longer contains platform entries.

Add _resolve_oci_platform_file() helper that handles both layouts:
- Flat: platform info directly in index.json (legacy/simple case)
- Nested: index.json → image index blob → platform manifests

All three multi-arch functions now use this single helper, eliminating
the layout resolution logic that would otherwise be duplicated in each.

Also fixes two issues in the vimport case block:
- 'local' keyword used outside a function (bash error on line 1879).
  The vimport handler is in a case statement in the main script body,
  not inside a function, so 'local' is invalid. The original multi-arch
  code was written assuming it would be inside a function.
- OCI_SELECTED_PLATFORM was blank in output because select_platform_manifest
  sets it inside a $() subshell, where variable assignments are lost.
  Use normalize_arch_to_oci directly for the display message instead.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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