Move per-query CAS state out of the `SwiftDependencyScanningService`

[artemcm]

Motivation

The global SwiftDependencyScanningService today is the owner of some shared state which would be better served by being lowered into the Scanner or Scanner workers.

Proposed solution

As a follow-up to #82429 consider also moving out things like the PrefixMapper out of the shared service.

Alternatives considered

No response

Additional information

No response