Implement architecture variants

Architecture variants are children of an architecture that share
the same ABI but are optimized for different ISA levels. They
are available in Ubuntu 25.10 and newer, and not supported in
Debian or other distributions.

A deb built for a variant contains the Architecture-Variant field,
and the Architecture field points to the baseline, for example:

    Architecture: amd64
    Architecture-Variant: amd64v3

However, the apt-get indextargets command reports the variant in the
Architecture: field, and most of the code in APT presents the variant
as the architecture.

There are two types of variants:

1. Standalone variants are recorded in the Architectures field of the
   Release file as if they were a real architecture:

        Architectures: amd64 amd64v3

   Standalone architecture variants only fetch the standalone
   architecture variant's Packages file. To do this, this patch
   changes the code such that the variants indextargets "supplant"
   the base targets.

   This may have complicated outcomes on the apt-get indextargets
   command.

2. Other variants can only be identified by their files being recorded
   with hashes in the Release file.

   APT fetches both the base architecture's as well as the variant's
   Packages file.

Variants are configured in the

    APT::Architecture-Variants

list.

Image builders may want to build specific variant images using

    APT::Architecture-Variants { "amd64v3"; }

But this commit also implements an automatic discovery mechanism
using the varianttable and /proc/cpuinfo.

    APT::Architecture-Variants "auto";

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