Manage @custom pipelines

Would there be a way to manage @custom pipelines in the package (e.g. by calling the file @custom.yml)?

This has as an advantage that you can easily modify an existing package, without having to change existing files, you'd just add @custom. Any other option requires to slightly modify the default.yml so it includes
a pipeline processor for a different pipeline in the package.

Managing the @custom pipelines in a separate way introduces overhead, as we have to deploy in a different way and 'breaks' the testing and benchmarking functionality as well.

Start by tracing how package pipelines are discovered and managed, focusing on the proposed @custom.yml and the existing default.yml pipeline-processor setup. Review the testing and benchmarking workflows described in the issue. Done means a package can add or modify a custom pipeline without changing existing files or requiring separate deployment, while testing and benchmarking continue to work.

Written by the indexing model from the issue text.