Update `ExportedImageTar.IndexLayerArchiveFactory` to support nested
indexes. Nested indexes support a layer of interaction where the
`index.json` file points to a blob that contains the read index to use.
Prior to this commit, we only supported indexes provided directly by
the `index.json` file. This missing support results in "buildpack.toml:
no such file or directory" errors when referencing specific buildpacks
and using Docker Engine 27.3.1 or above.
See gh-43126
a293560237