[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.

include/swift/Serialization/SerializedModuleLoader.h

@@ -35,6 +35,15 @@ enum class ModuleLoadingMode {
   OnlySerialized
 };
 
+/// How a dependency should be loaded.
+///
+/// \sa getTransitiveLoadingBehavior
+enum class ModuleLoadingBehavior {
+  Required,
+  Optional,
+  Ignored
+};
+
 /// Helper type used to pass and compute the sets of related filenames used by
 /// \c SerializedModuleLoader subclasses.
 struct SerializedModuleBaseName {
@@ -232,8 +241,11 @@ public:
 
   virtual void verifyAllModules() override;
 
-  virtual llvm::Optional<const ModuleDependencyInfo *>
-  getModuleDependencies(StringRef moduleName, ModuleDependenciesCache &cache,
+  virtual llvm::SmallVector<std::pair<ModuleDependencyID, ModuleDependencyInfo>, 1>
+  getModuleDependencies(StringRef moduleName, StringRef moduleOutputPath,
+                        llvm::IntrusiveRefCntPtr<llvm::cas::CachingOnDiskFileSystem> CacheFS,
+                        const llvm::DenseSet<clang::tooling::dependencies::ModuleID> &alreadySeenClangModules,
+                        clang::tooling::dependencies::DependencyScanningTool &clangScanningTool,
                         InterfaceSubContextDelegate &delegate,
                         bool isTestableImport) override;
 };