[Dependency Scanning] Pull optional dependencies from the adjacent binary module for direct interface dependencies

For a `@Testable` import in program source, if a Swift interface dependency is discovered, and has an adjacent binary `.swiftmodule`, open up the module, and pull in its optional dependencies. If an optional dependency cannot be resolved on the filesystem, fail silently without raising a diagnostic.

include/swift/AST/ModuleLoader.h

@@ -245,7 +245,8 @@ public:
   /// If a non-null \p versionInfo is provided, the module version will be
   /// parsed and populated.
   virtual bool canImportModule(ImportPath::Module named,
-                               ModuleVersionInfo *versionInfo) = 0;
+                               ModuleVersionInfo *versionInfo,
+                               bool isTestableImport = false) = 0;
 
   /// Import a module with the given module path.
   ///
@@ -328,7 +329,8 @@ public:
   virtual Optional<const ModuleDependencyInfo*> getModuleDependencies(
       StringRef moduleName,
       ModuleDependenciesCache &cache,
-      InterfaceSubContextDelegate &delegate) = 0;
+      InterfaceSubContextDelegate &delegate,
+      bool isTestableImport = false) = 0;
 };
 
 } // namespace swift