fix: avoid overwriting google-cloud-functions modules

[iamdhrv]

Summary

• Move the background function Context shim into functions_framework.
• Stop distributing packages under google.cloud, which overwrote SDK modules during co-installation.
• Update framework imports and conversion tests.

Fixes #184.

Testing

• python -m pytest -m "not slow_integration_test"
• black --check src tests conftest.py
• isort -c src tests conftest.py
• Installed alongside google-cloud-functions and verified functions_v1.CloudFunctionsServiceClient remains importable.
• Built the wheel and confirmed it contains no google/ paths.