perf: Defer heavy imports in ServiceLocator to reduce import time

[Lidang-Jiang]

Summary

Ref #1253 — Importing the crawler class takes over 2 seconds.

This is a partial optimization that addresses the ServiceLocator module's contribution to slow import times. The _service_locator.py module was eagerly importing heavy dependencies (Configuration, EventManager, LocalEventManager, FileSystemStorageClient, StorageClient, ServiceConflictError) at module level, even though these are only needed when services are first accessed.

Changes:

• Move Configuration, EventManager, LocalEventManager, FileSystemStorageClient, StorageClient, and ServiceConflictError imports from module level into the methods where they are used
• Add TYPE_CHECKING guards for type annotations that reference these classes
• No behavioral changes — all services are still lazily instantiated as before

Impact:

• import crawlee is ~73% faster (0.40s → 0.11s)
• Full from crawlee.crawlers import ParselCrawler import is not significantly affected because _basic_crawler.py eagerly imports many of the same modules — further optimization of that module could be a follow-up

The ServiceLocator docstring already states "All services are initialized to its default value lazily" — this change makes the imports consistent with that design intent.

Before

$ python -c "import time; start = time.perf_counter(); import crawlee; print(f'{time.perf_counter() - start:.4f}s')"
0.4220s
0.3898s
0.3837s

After

$ python -c "import time; start = time.perf_counter(); import crawlee; print(f'{time.perf_counter() - start:.4f}s')"
0.1188s
0.1069s
0.1123s

Test results (113 tests pass)

$ python -m pytest tests/unit/ -v --timeout=120 -x -k "service_locator or storage_client or configuration"

tests/unit/test_service_locator.py::test_default_configuration PASSED
tests/unit/test_service_locator.py::test_custom_configuration PASSED
tests/unit/test_service_locator.py::test_configuration_overwrite_not_possible PASSED
tests/unit/test_service_locator.py::test_configuration_conflict PASSED
tests/unit/test_service_locator.py::test_default_event_manager PASSED
tests/unit/test_service_locator.py::test_custom_event_manager PASSED
tests/unit/test_service_locator.py::test_event_manager_overwrite_not_possible PASSED
tests/unit/test_service_locator.py::test_event_manager_conflict PASSED
tests/unit/test_service_locator.py::test_default_storage_client PASSED
tests/unit/test_service_locator.py::test_custom_storage_client PASSED
tests/unit/test_service_locator.py::test_storage_client_overwrite_not_possible PASSED
tests/unit/test_service_locator.py::test_storage_client_conflict PASSED
... (+ 101 storage_client and configuration tests)

==================== 113 passed, 1574 deselected in 14.90s =====================

Test plan

• All 12 test_service_locator.py tests pass
• All 113 service_locator/storage_client/configuration tests pass
• ruff check passes
• import crawlee benchmark shows ~73% improvement
• CI passes