refactor: flatten deeply nested logic with early returns and guard clauses - #368

Summary

Flattens deeply nested control flow (if/else pyramids) across pycross/private/ and modules/backend_maturin/ using early returns, guard statements with continue, and small focused helper functions:

• Lock translation (translator_common.bzl, uv_lock_model.bzl, poetry_lock_model.bzl, pylock_lock_model.bzl):
  • Extract _read_project_dict and _resolve_local_dep_constraint in translator_common.bzl using early returns and continue guards; flatten package deduplication, dependency matching, and pin resolution loops in resolve_lock_graph.
  • Extract _resolve_uv_dep_constraint and _parse_sdist_file in uv_lock_model.bzl, and flatten _resolve_package_requires_python, _parse_uv_dependency, and group/member iteration with continue guards.
  • Extract _resolve_pylock_project_pins in pylock_lock_model.bzl and flatten include-group, BFS reachability, and fork-pin construction loops.
  • Flatten _get_files_for_package and group filtering in poetry_lock_model.bzl.
• Lock resolution, rendering & repo generation (lock_resolver.bzl, lock_common.bzl, lock_repo_creation.bzl, resolved_lock_renderer.bzl, thin_package_repo.bzl, sdist_repo.bzl, pypi_file.bzl, format_extension.bzl, backends.bzl):
  • Extract _synthesize_extra_package in lock_resolver.bzl and use continue guards in _create_package_resolver and _compute_cycle_groups.
  • Extract _create_remote_file_repo in lock_repo_creation.bzl and _extract_href in pypi_file.bzl.
  • Flatten workspace validation and rendering loops in lock_common.bzl, format_extension.bzl, backends.bzl, resolved_lock_renderer.bzl, thin_package_repo.bzl, and sdist_repo.bzl.
• Cycle, marker & runtime rules (cycle_dep_needed.bzl, cycle_member_marker_deps.bzl, pep508_marker_values.bzl, wheel_library.bzl, modules_mapping.bzl, build/rules/common_attrs.bzl):
  • Extract _register_wheel_install_action and _build_venv_symlinks in wheel_library.bzl, _node_env in cycle_dep_needed.bzl, and _toolchain_py3_runtime in pep508_marker_values.bzl.
  • Flatten loop conditions in cycle_member_marker_deps.bzl, modules_mapping.bzl, and common_attrs.bzl.
• Python tools (inspect_package.py, cc_toolchain.py, generate_cargo_lock.py, rust_common.py):
  • Extract _find_cargo_toml_dir_from_pyproject in generate_cargo_lock.py (with open(pyproject_path) inside the fallback try/except) and _symlink_stdlib in rust_common.py.
  • Use early returns and continue guards in inspect_package.py and cc_toolchain.py (get_wrapper_flags now matches exact flag names via partition("=") before reading split_cflags[i + 1], avoiding an IndexError if the last flag only shares a prefix like --sysroots).