[Dependency Scanning] Refactor primary scan operations into 'ModuleDependencyScanner' class

From being a scattered collection of 'static' methods in ScanDependencies.cpp
and member methods of ASTContext. This makes 'ScanDependencies.cpp' much easier
to read, and abstracts the actual scanning logic away to a place with common
state which will make it easier to reason about in the future.

lib/Serialization/ModuleFileSharedCore.h

@@ -24,15 +24,10 @@ namespace llvm {
 }
 
 namespace swift {
+enum class ModuleLoadingBehavior;
+}
 
-/// How a dependency should be loaded.
-///
-/// \sa getTransitiveLoadingBehavior
-enum class ModuleLoadingBehavior {
-  Required,
-  Optional,
-  Ignored
-};
+namespace swift {
 
 /// Serialized core data of a module. The difference with `ModuleFile` is that
 /// `ModuleFileSharedCore` provides immutable data and is independent of a