Compute preliminary overlay database mode

[cklin]

This PR updates the init action to compute a preliminary overlay database mode before the CodeQL CLI becomes available. The preliminary overlay database mode will be used in a future PR.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[cklin]

Hi @mbg,

Thank you for your detailed comments. Before we go into the details, can you confirm that I understand your main concerns correctly?

1. You are concerned that this change is high-risk because it is a fairly big change, not under feature flags, and there is insufficient test coverage.
2. You are concerned that there is too much code duplication between getPreliminaryOverlayDatabaseMode() and the rest of the init action, and that the behavior of getPreliminaryOverlayDatabaseMode() is inconsistent with the main init action logic.
3. You are concerned that there is insufficient logical separation between getPreliminaryOverlayDatabaseMode() and the rest of the init action (for example, with initActionState()), and that the hidden interactions will cause maintenance burden down the road.

Do I understand your main concerns correctly? Am I missing anything?