fix: fall back to setuptools when declared sdist build backend is unavailable

[xangcastle]

When building a wheel from an sdist, the build venv only contains the packages listed in default_build_dependencies (typically build, setuptools, and wheel). If a package declares a different build backend in its pyproject.toml (hatchling, flit, poetry-core, etc.) the python -m build --no-isolation invocation fails immediately because the backend module is not importable.

This adds ensure_build_backend() to a new build_backend.py module (imported by build_helper.py) that detects this situation before the build starts: if the declared backend is not importable it rewrites the [build-system] table to use setuptools.build_meta, and for environments running setuptools < 61.0 (which predates PEP 621 [project] table support) it also generates a setup.cfg from the parsed project metadata so the build can proceed without a modern setuptools.

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Changes are visible to end-users: no

Test plan

• New test cases added

[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Thu Jun 11 15:54:11 UTC 2026

❌ 6 failed tasks

• ❌ buildifier · ⏱ 19.9s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in diff · Format failed (1 file needs format)
• ❌ gazelle · ⏱ 26.5s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in diff · Gazelle failed (1 out of date)
• ❌ test (test-e2e-bazel-8) · ⏱ 31m 3s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed
• ❌ test (test-e2e-bazel-9) · ⏱ 18m 10s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed
• ❌ test (test-root-bazel-8) · ⏱ 24m 9s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed
• ❌ test (test-root-bazel-9) · ⏱ 30m 9s · 🐙 GitHub Actions · ☑️ Check
  💬 failed in test · Bazel test failed

✅ 1 successful task

• ✅ test (test-examples-uv_pip_compile-bazel-8) · ⏱ 27.6s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed · 1 cached)

🛠️ Fix

❌ buildifier (buildifier)

aspect buildifier --severity=info -- uv/private/pep517_whl/BUILD.bazel

# without Aspect CLI
bazel run --run_in_cwd @buildifier_prebuilt//buildifier -- uv/private/pep517_whl/BUILD.bazel

❌ gazelle (gazelle)

aspect gazelle --severity=info --check-only=false -- uv/private/pep517_whl

# without Aspect CLI
bazel run //tools/gazelle:gazelle

_{Install aspect: docs.aspect.build/cli/install}

🔁 Reproduce

❌ buildifier (buildifier)

aspect buildifier --severity=fail --base-ref=origin/1.x

❌ gazelle (gazelle)

aspect gazelle --severity=fail --check-only=true

❌ test (test-e2e-bazel-8 · test-e2e-bazel-9)

aspect test -- \
  @@aspect_rules_py++uv+sdist_build__googlemaps_test__googlemaps__4_10_0//:whl \
  //cases/uv-legacy-deps-750:googlemaps

# without Aspect CLI
bazel test -- \
  @@aspect_rules_py++uv+sdist_build__googlemaps_test__googlemaps__4_10_0//:whl \
  //cases/uv-legacy-deps-750:googlemaps

❌ test (test-root-bazel-8 · test-root-bazel-9)

aspect test -- \
  @@+uv+sdist_build__aspect_rules_py__bravado_core__6_1_1//:whl \
  //examples/pytest:coverage_lcov_shape_test

# without Aspect CLI
bazel test -- \
  @@+uv+sdist_build__aspect_rules_py__bravado_core__6_1_1//:whl \
  //examples/pytest:coverage_lcov_shape_test

_{Install aspect: docs.aspect.build/cli/install}

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