fix: expose private EXECUTABLE_ATTRS defaults to external rule builders

[13steinj]

Why

create_executable_rule_builder() (used by py_binary_rule_builder() /
py_test_rule_builder(), the public python/api/executables.bzl API) bundles
three implicit attrs whose defaults point at private targets under
//python/private: build_data_writer, debugger_if_target_config, and
uncachable_version_file. These targets had no explicit visibility, so they
inherited the package's default_visibility (//:__subpackages__), which
only covers packages inside the rules_python repo itself.

Because the builder API is designed to let external modules call rule()
themselves (via builder.build()), Bazel checks visibility of these attr
defaults from the calling module's package, not from rules_python's. Any
external repo constructing a rule via py_binary_rule_builder() /
py_test_rule_builder() therefore fails at analysis time:

ERROR: <BUILD file>: in <custom_rule> rule <target>: Visibility error:
alias '@@rules_python+//python/private:build_data_writer' referring to target
'@@rules_python+//python/private:build_data_writer.sh' is not visible from
target '<calling .bzl file>'

(and the same for debugger_if_target_config and uncachable_version_file).

This has been broken since the builder API's introduction; there's prior art
in this same file for exactly this situation — stage1_bootstrap_template,
stage2_bootstrap_template, site_init_template, bootstrap_template, and
current_interpreter_executable are all public for the identical reason
(implicit deps of py_runtime), each with a
# Not actually public. Only public because it's an implicit dependency of ...
comment.

Before / after

Before: any external module using py_binary_rule_builder() /
py_test_rule_builder() fails at analysis time with the visibility error
above.

After: it analyzes and builds successfully.

Changes

• python/private/BUILD.bazel: mark build_data_writer,
  debugger_if_target_config, uncachable_version_file (plus empty and
  sentinel, which the alias chains resolve through) //visibility:public,
  matching the existing comment convention for implicit-dependency targets.
• python/private/uncachable_version_file.bzl: define_uncachable_version_file()
  is a macro, not a rule(), so it doesn't get a visibility kwarg for free —
  added one and forwarded it to the underlying native.alias().
• examples/bzlmod/other_module/other_module/rule_builder/: new fixture — a
  minimal custom py_binary built via py_binary_rule_builder() from
  other_module (a separate bzlmod module, simulating a real external
  consumer). Verified this fails on the parent commit and passes with the fix.
• examples/bzlmod/tests/other_module/BUILD.bazel: build_test wiring the
  fixture into other_module's existing CI-covered //... build.

Manually verified in an isolated minimal repro (bare BCR, git_override
pointing at this branch) as well as against the examples/bzlmod example
included in this PR.

[gemini-code-assist]

Code Review

This pull request resolves a visibility issue where implicit attribute defaults used by py_binary_rule_builder() and py_test_rule_builder() were private to rules_python, causing analysis failures for external modules. The changes expose these targets as public and add regression tests. The review feedback suggests forwarding the visibility parameter to uncachable_version_file_impl to prevent failures when stamping is enabled, and cleaning up placeholder TODO(pr-number) comments in the test files.

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[gemini-code-assist]

When --stamp is enabled, the uncachable_version_file alias resolves to :uncachable_version_file_impl. Since :uncachable_version_file_impl is a rule target and remains private (inheriting the package's default visibility), external rule builders will fail with a visibility error during analysis when stamping is active. Forward the visibility parameter to uncachable_version_file_impl as well to ensure it is visible to external consumers.

Suggested change

	def define_uncachable_version_file(name, visibility = None):
	native.alias(
	name = name,
	actual = select({
	":stamp_detect": ":uncachable_version_file_impl",
	"//conditions:default": ":sentinel",
	}),
	visibility = visibility,
	)
	uncachable_version_file(name = "uncachable_version_file_impl")
	def define_uncachable_version_file(name, visibility = None):
	native.alias(
	name = name,
	actual = select({
	":stamp_detect": ":uncachable_version_file_impl",
	"//conditions:default": ":sentinel",
	}),
	visibility = visibility,
	)
	uncachable_version_file(
	name = "uncachable_version_file_impl",
	visibility = visibility,
	)

[gemini-code-assist]

Avoid leaving unresolved TODO(pr-number) placeholders in comments. We can describe the regression coverage more generally without referencing a specific PR number.

Suggested change

	Regression coverage for https://github.com/bazel-contrib/rules_python/pull/3919:
	Regression coverage for the executables rule builder API visibility issue:

[gemini-code-assist]

Avoid leaving unresolved TODO(pr-number) placeholders in comments. We can simplify the comment to refer to the regression test generally.

Suggested change

	# Regression test for https://github.com/bazel-contrib/rules_python/pull/3919: py_binary_rule_builder()'s implicit
	# Regression test: py_binary_rule_builder()'s implicit