Instead of exposing just the partition type UUID, let's expose the
GptPartitionType struct, which has a lot more information available
in a much more accessible way.

Also, let's get rid of SECONDARY/OTHER in PartitionDesignator. These
were only there to support preferred architectures in dissect-image.c,
but we can easily handle that by comparing architectures when we decide
whether to override a partition. This is done in a new function
compare_arch().