[Dependency Scanning] Add a C API layer for dependency scanning tool, scanning actions, and scan results.

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.

include/swift-c/DependencyScan/module.modulemap

@@ -0,0 +1,4 @@
+module _InternalDependencyScan {
+  header "DependencyScan.h"
+  link "_InternalDependencyScan"
+}