Adds a C API layer consisting of:
- Data structures used to represent in-memory result of dependency scanning
- Opaque dependency scanner tool (C wrapper for `DependencyScanningTool`)
Refactors `ScanDependencies.cpp` to produce dependency scanning result in the form of the above binary format.
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
