Fix powerpack dependency ID remapping

[riyazsh]

Summary

• remap monitor and SLO IDs under the actual powerpack group_widget payload shape
• keep existing top-level widget paths for compatibility
• add unit coverage for powerpack ID remapping and top-level identity/author stripping

Why

Powerpacks wrap dashboard widgets under attributes.group_widget, but sync-cli only had dashboard-style widgets.* dependency paths. That means monitor/SLO references inside powerpacks could remain as source-org IDs in the destination copy.

Testing

• tox -e py -- --black datadog_sync/model/powerpacks.py tests/unit/test_powerpacks.py tests/unit/test_import_force_missing_deps.py::test_extract_source_ids_overrides_complete