Report cannot load submodule if only able to load top module

For loaders that don't support loading submodules, canImportModule should report false instead of checking if can import top module.

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1849,6 +1849,8 @@ bool ExplicitSwiftModuleLoader::canImportModule(ImportPath::Module path,
                                                 llvm::VersionTuple version,
                                                 bool underlyingVersion) {
   // FIXME: Swift submodules?
+  if (path.hasSubmodule())
+    return false;
   ImportPath::Element mID = path.front();
   // Look up the module with the real name (physical name on disk);
   // in case `-module-alias` is used, the name appearing in source files