Address PR #942 review comments + buildifier fix

[thesayyn]

Addresses review comments on #942 and the buildifier CI failure. Targets new_layering so the changes flow into the existing PR.

Changes

• normalize_label (per @jbedard): replaced the hardcoded aspect_rules_py++uv+whl_install__ prefix match with an _extract_whl_install_pkg helper anchored on the stable whl_install__ substring, so the same code path works under both Bazel 8 (+) and Bazel 9 (~) module-extension separators.
• _py_image_layer_impl (per @akafael):
  • Collapsed seen_labels + all_pkgs list + pkg_by_label rebuild into a single dedup-by-label dict pass; all_pkgs = pkg_by_label.values().
  • Dropped the dep_tars double-bookkeeping inside the loops — snapshot dep_tars = list(all_tars) once before the source layer is appended.
• py/private/BUILD.bazel (per @jbedard): dropped the @rules_python//python:defs.bzl load; the validator now uses rules_py's own py_binary rule.
• py/tests/internal-deps/adder/BUILD.bazel (per @jbedard): reverted visibility back to the prior explicit list. Flagging that this may break e2e/cases/oci/{py_image_layer,py_venv_image_layer} consumers — happy to widen the list if CI complains.
• Buildifier: blank line at the interpreter-layer block, multi-line _declare_group_tar calls split per arg.

Test plan

• CI green on Bazel 8 and Bazel 9 (the bazel-9 separator fix should unblock the multi-member group path under Bazel 9).
• //cases/interpreter-features-836:test and //cases/oci/py_venv_image_layer:py_amd64_image_command_test — the venv command test was Bazel-9-only and may resolve via the separator fix; the interpreter-features test is independent and worth checking buildkite logs for.

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[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

123 tests (100.0%) were fully cached saving 60s.

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Bazel 9 (Test)

All tests were cache hits

122 tests (100.0%) were fully cached saving 1m 5s.

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Bazel 8 (Test)

e2e

1 test target passed

Targets

//cases/interpreter-features-836:test [k8-fastbuild]5s

Total test execution time was 5s. 66 tests (98.5%) were fully cached saving 1m 10s.

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Bazel 9 (Test)

e2e

⚠️ Buildkite build #3378 failed.

Failed tests (2)

//cases/interpreter-features-836:test [k8-fastbuild]🔗
//cases/oci/py_venv_image_layer:py_amd64_image_command_test [k8-fastbuild]🔗

💡 To reproduce the test failures, run

bazel test //cases/interpreter-features-836:test //cases/oci/py_venv_image_layer:py_amd64_image_command_test

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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Buildifier