System Buildpacks

[jabrown85]

Summary

This pull request introduces support for merging system buildpacks (defined in system.toml) into the buildpack order for Platform API >= 0.15. It includes changes to the detector phase, CLI flags, configuration handling, and adds comprehensive tests to verify the new behavior. The most important changes are grouped below.

Platform API 0.15+ system buildpack support:

• The detector phase now reads and merges system buildpacks (from system.toml) with the buildpack order when Platform API >= 0.15. System buildpacks are prepended and appended to each group in the order. [1] [2] [3] [4]
• Added CLI flag --system to specify the path to system.toml for relevant commands, and enabled it for Platform API >= 0.15. [1] [2] [3]

Configuration and handler updates:

• The ConfigHandler interface now supports reading system buildpacks via a new ReadSystem method.
• Updated the detector factory to use the new getOrderWithSystem method, which merges system buildpacks as needed.

Testing and validation:

• Added new acceptance and unit tests for merging system buildpacks, covering various scenarios including only pre/post buildpacks, empty system, and extension preservation. [1] [2] [3]
• Added test data for system.toml and middle_order.toml to support acceptance tests. [1] [2]

Minor code cleanup:

• Reordered imports in several test mock files for consistency. [1] [2] [3] [4] [5] [6]

Release notes

• Added support in Platform 0.15 for System Buildpacks.

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Related

Implements Spec Changes here

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Context

• Spec PR has been merged
• Any updates to spec PR are reflected in changesets

[jabrown85]

buildpacks/spec#419 has been merged. Once 0.15 is cut, this can be merged in and released.