Use explicit __init__.py files instead of implicit creation

[onnosteenbergen-samotics]

Fixes #1081.

rules_python warns for executables that rely on implicit __init__.py creation (bazel-contrib/rules_python#2945), which shows up for every consumer of pkg_tar, pkg_zip, pkg_deb, pkg_install and verify_archive_test.

• Add the missing pkg/private/{tar,deb}/__init__.py and put every __init__.py in the srcs of a py_library (//pkg:init, //pkg/private:init).
• Set legacy_create_init = 0 on all py_binary/py_test targets under pkg/, including those created by the macros. The per-target attribute is used because the module-wide explicit_init_py setting requires rules_python 2.3.
• Add //tests:explicit_init_test, which fails if a package is imported as a namespace package.

🤖 Generated with Claude Code

[google-cla]

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[onnosteenbergen-samotics]

I've signed the CLA, but the co-author is Claude.ai. Don't know how to solve that one.

[tonyaiuto]

Have you tried this without setting legacy_create_init? ISTM that it should still work without that, and be backwards compatible with older python rules

[aiuto]

I've signed the CLA, but the co-author is Claude.ai. Don't know how to solve that one.

The only author that matters is onnosteenbergen-samotics.
The check is failling. Did you use that github identity?

[onnosteenbergen-samotics]

Re-ran the check and it still marks Claude as failed.

Will verify if legacy_init_create = 0 is required, would indeed make sense to leave it out.

[onnosteenbergen-samotics]

┌─────────────────────────────────────────────────────────────────────┬──────────────┬──────────┐
│                          rules_pkg variant                          │ rules_python │ Warnings │
├─────────────────────────────────────────────────────────────────────┼──────────────┼──────────┤
│ with legacy_create_init = 0                                         │ 2.3.2        │ 0        │
├─────────────────────────────────────────────────────────────────────┼──────────────┼──────────┤
│ without                                                             │ 2.3.2        │ 5        │
├─────────────────────────────────────────────────────────────────────┼──────────────┼──────────┤
│ with                                                                │ 1.7.0        │ 0        │
├─────────────────────────────────────────────────────────────────────┼──────────────┼──────────┤
│ without                                                             │ 1.7.0        │ 0        │
├─────────────────────────────────────────────────────────────────────┼──────────────┼──────────┤
│ without, consumer passes --incompatible_default_to_explicit_init_py │ 2.3.2        │ 0        │
└─────────────────────────────────────────────────────────────────────┴──────────────┴──────────┘

The 5 warnings in the "without" run on 2.3.2:

WARNING: Target @@rules_pkg+//pkg/private/tar:build_tar is using implicit __init__.py creation.
WARNING: Target @@//:demo_install is using implicit __init__.py creation.
WARNING: Target @@rules_pkg+//pkg/private/zip:build_zip is using implicit __init__.py creation.
WARNING: Target @@//:demo_tar_test is using implicit __init__.py creation.
WARNING: Target @@rules_pkg+//pkg/private/deb:make_deb is using implicit __init__.py creation.

The tar, zip and deb outputs had the same sha256 in every run (e.g. demo_tar.tar is 0639fb21…).

• The warning doesn't look at the files. rules_python 2.3.2 decides only from the target's legacy_create_init, then the owning module's explicit_init_py config, then the CLI flag. Whether __init__.py files exist doesn't matter, so this might be something that rules_python needs to fix.
• rules_pkg could set explicit_init_py in its own MODULE.bazel, but that requires rules_python 2.3 or newer as a minimum. The attribute works on 1.7.0, the version rules_pkg currently requires.
• Without the attribute, users can silence the warning themselves with the flag (last row of the table). Before this PR, that flag didn't make the packages proper Python packages.