Create a multi-arch buildpackage file instead of a buildpackage file for each target

[ForestEckhardt]

Description

When creating a buildpackage file for a multi-arch buildpack multiple buildpackage files will be created one for each architecture that is supported by the buildpack.

Proposed solution

When creating a buildpackage file for a multi-arch buildpack to create a single buildpackage file in the same way that you can create a single multi-arch OCI image. This will mean that the artifact that is published and file that is created will behave identically allowing for the continued parity between the .cnb file and the images that are uploaded to the registry.