ModuleInterface: refactor ModuleInterfaceChecker out of ModuleInterfaceLoader

This refactoring allows us to drop ModuleInterfaceLoader when explicit modules
are enabled. Before this change, the dependencies scanner needs the loader to be
present to access functionalities like collecting prebuilt module candidates.

lib/Serialization/ModuleDependencyScanner.cpp

@@ -98,8 +98,7 @@ std::error_code PlaceholderSwiftModuleScanner::findModuleFilesInDirectory(
 static std::vector<std::string> getCompiledCandidates(ASTContext &ctx,
                                                       StringRef moduleName,
                                                       StringRef interfacePath) {
-  return static_cast<SerializedModuleLoaderBase*>(ctx
-    .getModuleInterfaceLoader())->getCompiledModuleCandidatesForInterface(
+  return ctx.getModuleInterfaceChecker()->getCompiledModuleCandidatesForInterface(
       moduleName.str(), interfacePath);
 }