Use importlib.util to locate fork specs, dropping pkg_resources

[lucasccordeiro]

Problem

ethcheck crashes at startup on environments with setuptools >= 81:

ModuleNotFoundError: No module named 'pkg_resources'
  File ".../ethcheck/ethcheck.py", line 15, in <module>
    import pkg_resources

pkg_resources was deprecated and is no longer bundled with setuptools
(removed in 81+). ethcheck.py imported it at module load and used
pkg_resources.resource_filename in get_file_path, so every entry point
(--help, --list-forks, --fork, ...) failed immediately.

Fix

Replace pkg_resources.resource_filename with the stdlib
importlib.util.find_spec (Python 3.9+). It resolves the fork's spec source
path from the package directory without importing the fork body, which
also decouples the core verification path from eth2spec's heavy runtime
dependencies.

spec = importlib.util.find_spec(module_name)
pkg_dir = spec.submodule_search_locations[0]
resource_path = os.path.join(pkg_dir, 'mainnet.py')  # falls back to spec.py

Verification

• ethcheck --help and --list-forks run (6 forks listed).
• get_file_path resolves the correct mainnet.py for all 6 forks
  (phase0, altair, bellatrix, capella, deneb, electra).
• Existing test suite: 9 passed.
• No new lint findings on the changed lines.

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