Add bzlmod support for arch_alias with module extension

[mmorel-35]

Fix arch_alias compatibility with bzlmod

clang_platform is not compatible with bzlmod: https://github.com/envoyproxy/envoy/actions/runs/20279190616/job/58236274568

Analysis

The arch_alias repository rule in bazel/repository/utils.bzl was created to enable architecture-specific platform aliases for Envoy. It works by:

• Creating an external repository with a BUILD file containing an alias
• Allowing .bazelrc to reference platform aliases like @clang_platform
• Resolving to different platforms based on host CPU architecture (amd64/aarch64)

Problem

The current implementation is WORKSPACE-only and doesn't follow bzlmod best practices:

• Repository rules in bzlmod need module extensions
• Should use proper tag classes for configuration
• Current approach doesn't work when --enable_bzlmod is set

Solution Implemented

• Analyze the arch_alias implementation and usage
• Create a module extension wrapper for arch_alias
• Add bzlmod-compatible extension definition with tag_class
• Fix alias naming in bzlmod to extract apparent name from canonical name
• Update documentation with bzlmod usage examples
• Create BCR-compatible test modules following best practices
• Organize tests properly: group under arch_alias/ and remove redundant _test suffix
• Remove unused alias_name parameter - the logic automatically handles bzlmod canonical names
• Add tests to GitHub workflow for both bzlmod and workspace modes
• Add test module to BCR presubmit configuration
• Remove module() declaration from test module (not intended for publication)
• Test both WORKSPACE and bzlmod modes
• Verify backward compatibility
• Address all code review feedback

Changes Made

1. Enhanced utils.bzl - Added arch_alias_ext module extension with proper tag_class
2. Automatic Name Extraction - The _get_alias_name() helper automatically extracts apparent names from bzlmod's canonical format (module++ext+name → name)
3. Removed Redundant Parameter - Removed alias_name parameter as it was unnecessary
4. Improved Documentation - Added inline documentation explaining bzlmod canonical vs apparent names
5. Well-Organized Test Structure:
   • bazel/test/arch_alias/bzlmod/ - BCR-compatible bzlmod test module
   • bazel/test/arch_alias/workspace/ - WORKSPACE mode backward compatibility tests
   • Tests grouped under common directory, no redundant _test suffix
   • Comprehensive README files documenting structure and usage
6. CI Integration:
   • Added arch_alias tests to GitHub workflow (.github/workflows/_bazel.yml) for both bzlmod and workspace modes
   • Added test_arch_alias task to BCR presubmit configuration (.bcr/bazel/presubmit.yml)
   • Removed module() declaration from test MODULE.bazel as it's not intended for publication

Testing

All tests pass in both bzlmod and WORKSPACE modes, verifying multiple aliases work correctly. Tests now run automatically in CI/CD pipelines on every PR and in BCR presubmit checks.

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[phlax]

thanks for working on this @mmorel-35

can you separate all the test changes out to a different pr please - lets make the change and then consider if/where to test as an external repo

(and remove all the bot docs - not against adding docs but they need to be minimal, focused and targeted and easy to maintain)

[phlax]

the test files here seem misplaced - altho i understand these tests need a separate workspace to use toolshed externally, which i think is not a bad idea - but then it should be testing non-bzlmod also

[phlax]

not sure iiuc but what is the point of the test if they all return the same thing

[phlax]

please dont keep submitting voluminous bot docs - it overwhelms reviewers and detracts from the actual changes