Generate implicit aliases for nested relative paths - #2233

What changed

• Generate implicit relative-path aliases for every directory component instead of only the first component.
• Add a regression test for a combined path whose usable local source is a deeper existing suffix.

Why

With relative_files = true, the first relative filename seen during combine could generate an alias such as */work -> work/. If that target did not exist locally, the path stayed unchanged even when a deeper suffix such as src/pkg/module.py did exist. Because input file ordering can vary, this produced intermittent No source for code errors.

Fixes #2195

Tests

• python -m pytest tests/test_files.py::PathAliasesTest::test_implicit_relative_path_uses_existing_suffix -q
• python -m pytest tests/test_files.py -q
• tox -q -e py313 (C tracer: 1592 passed; Python tracer: 1561 passed; sys.monitoring: 1542 passed)
• tox -q -e lint
• tox -q -e mypy
• pre-commit run --files coverage/files.py tests/test_files.py