Fix duplicate custom task module imports

[yangweigbh]

Summary

• reuse an already imported custom task module when its resolved file path matches
• avoid rerunning module-level metric and task registration code
• add regression coverage for preloaded and newly loaded custom task files

Fixes #805

Testing

• pytest tests/unit/tasks/test_registry.py (10 passed)
• ruff check src/lighteval/tasks/registry.py tests/unit/tasks/test_registry.py
• ruff format --check src/lighteval/tasks/registry.py tests/unit/tasks/test_registry.py