Support hpack includes

Hpack supports "!include" directives to inline the source of other yaml files. See: https://github.com/sol/hpack?tab=readme-ov-file#not-repeating-yourself

This raises two issues:
- "!include" is a non-standard extension of YAML and isn't supported by "remarshal" so the `readYAML` function will fail.
- Usually you will have the included YAML files at the top-level or in a different sub-directory to the `package.yaml` file and we currently don't have a way to provide this

I have a potential solution and I will create an MR soon.