Bundling for OpenAPI specification has long been a need for authors to allow to reduce duplication, and to allow to split a big specification in multiples files, but publish a single one.

A few years ago I've written a tool to fit that niche: https://github.com/dolmen-go/openapi-preprocessor

https://github.com/dolmen-go/openapi-preprocessor

I have now to tweak it (well, it will be a major rewrite) to handle $ref relative to $id instead of the file location.