Add <layers>/order.toml to path resolution

[jabrown85]

Allowing <layers>/order.toml to resolve before a builder's /cnb/order.toml will let platforms control the buildpacks to be executed from inside the same pod. A platform may mount <layers> to a container that executes prior to /cnb/detector and can write an order.toml to be consumed by /cnb/detector without knowing anything about the builder to be used.

[hone]

+1 on the idea of this change. My only other concern (also discussed in the linked RFC) is where this should live. Is it common for the platform to be writing things to the <layers>?

[jabrown85]

+1 on the idea of this change. My only other concern (also discussed in the linked RFC) is where this should live. Is it common for the platform to be writing things to the <layers>?

I put it at <layers> mostly for convenience and future proofing. I'm a bit concerned that /platform may be mounted read-only and that any future preparer stage that parses a project.toml would be unable to write an order.toml in that directory.

[hone]

We talked about re-organizing where we store things. I don't think it should block this spec change, but we're definitely coming to a point where we need to deal with that sooner than later.