This commit refactors ScanDependencies.cpp to split the functionality into two functional groups:
- Scan execution code that performs the mechanics of the scan and produces an in-memory result
- Dependency scanner entry-points used when the scanning action is invoked as a `swift-frontend` execution mode
This commit also adds the aforementioned in-memory dependency scanning result in `FullDependencies.h`, modeled after the InterModuleDependencyGraph representation in swift-driver
