libcnb.LayerContributor => libpak.Contributable

[BarDweller]

Summary

libcnb 2.x removed libcnb.LayerContributor, a concept that was useful for libpak, and consumers of libpak.

This PR adds 'libpak.Contributable' a replacement for the old libcnb.LayerContributor interface, that has identical methods, allowing it to be used almost seamlessly where-ever the old LayerContributor was used.

"Almost" because any buildpacks previously returning an array of libcnb.LayerContributor now need to return an array of libcnb.Layer which means the Contributables must be made to Contribute before the return has the correct data. To this end, a a Function type (FlattenContributableFn), and default implementation (DefaultFlattenContributableFn) are supplied to make this easier for buildpacks to perform this conversion.

Use Cases

Allows buildpacks / libpak consumers to continue to use libcnb.LayerContributor like concepts, with minimal impact.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).