Bazel rule: detect "oci_layout" and "oci_tarball" output groups

[malt3]

With rules_oci, the output of "oci_image" and "oci_image_index" is always a tree artifact containing the full oci layout. rules_img is an upcoming Bazel ruleset for building container images that tries to avoid unnecessary work. To that end, rules_img will only materialize a full oci layout tree artifact if a downstream action needs it. This is done through the use of two special output groups:

• oci_layout contains a tree artifact of the full oci layout
• oci_tarball contains is a tar file containing the oci layout

With this change, container_structure_test is compatible with both rulesets. Users can continue to pass a single file or can choose to use the aforementioned output groups.

This results in a better user experience for rules_img users who otherwise need to create a special intermediate filegroup to convert from OutputGroupInfo to DefaultInfo.

[gemini-code-assist]

Summary of Changes

Hello @malt3, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the container_structure_test Bazel rule by introducing support for the oci_layout and oci_tarball output groups, which are integral to the rules_img ruleset. The primary goal is to improve compatibility and user experience by allowing the rule to directly consume these specialized output groups, thereby eliminating the need for manual intermediate steps and ensuring seamless integration with modern OCI image build workflows.

Highlights

• Enhanced OCI Output Group Detection: The container_structure_test Bazel rule now explicitly detects and utilizes oci_layout and oci_tarball output groups, which are introduced by the upcoming rules_img ruleset for building container images.
• Improved Compatibility: This change ensures container_structure_test is compatible with both rules_oci and rules_img, providing a unified testing experience across different OCI image generation methods.
• Streamlined User Experience: Users of rules_img no longer need to create intermediate filegroup targets to convert OutputGroupInfo to DefaultInfo, simplifying their Bazel configurations.
• Flexible Image Attribute Handling: The image attribute in the container_structure_test rule has been updated to allow_files = True and now intelligently extracts the relevant image file from either specific OCI output groups or the DefaultInfo provider.

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[gemini-code-assist]

Code Review

This pull request enhances the container_structure_test Bazel rule by adding support for oci_layout and oci_tarball output groups, which is useful for compatibility with rules_img. The implementation correctly checks for these output groups and falls back to the previous behavior of using a single file from DefaultInfo. The overall change is good, but there is an opportunity to refactor the new logic to improve its clarity and reduce code duplication.