chore: make pipstar non-switchable

[aignas]

The pipstar code has been there for some time and might be stable enough to not
have a fallback anymore.

Related #2949

[gemini-code-assist]

Code Review

This pull request removes the RULES_PYTHON_ENABLE_PIPSTAR and RULES_PYTHON_ENABLE_PYSTAR environment variables and their associated logic, effectively making the Starlark implementations the default. It also removes the legacy entry point generation mechanism in favor of py_console_script_binary. Feedback was provided regarding a potential breaking change in the whl_library_targets macro signature, where retaining the entry_points parameter is recommended for backward compatibility with vendored files. Additionally, a null check was suggested for the whl_metadata result in whl_library.bzl to prevent potential crashes if the metadata file is missing.

[gemini-code-assist]

Removing the entry_points argument from the whl_library_targets macro is a breaking change for users who have vendored requirements.bzl files or are using custom BUILD file templates that still pass this argument. To maintain backward compatibility and avoid a TypeError in Starlark, it is recommended to keep the argument in the signature but ignore its value.

        data = [],
        copy_files = {},
        copy_executables = {},
        entry_points = {},
        native = native,
        enable_implicit_namespace_pkgs = False,
        namespace_package_files = [],

[gemini-code-assist]

The whl_metadata function can return None if it fails to find or parse the METADATA file (as seen in whl_metadata.bzl). If it returns None, the subsequent access to metadata.name at line 444 will cause a crash with a potentially confusing error message. Although logger.fail is called inside whl_metadata, adding an explicit check here prevents a secondary crash and provides a cleaner exit from the repository rule.

Suggested change

	metadata = whl_metadata(
	install_dir = install_dir_path,
	read_fn = rctx.read,
	logger = logger,
	)
	metadata = whl_metadata(
	install_dir = install_dir_path,
	read_fn = rctx.read,
	logger = logger,
	)
	if not metadata:
	return None