Consistently use variable mangledScopedPackageSeparator instead of magic "__" string (microsoft#16713)

Author: Andy

src/compiler/moduleNameResolver.ts

@@ -995,7 +995,7 @@ namespace ts {
     export function getPackageNameFromAtTypesDirectory(mangledName: string): string {
         const withoutAtTypePrefix = removePrefix(mangledName, "@types/");
         if (withoutAtTypePrefix !== mangledName) {
-            return withoutAtTypePrefix.indexOf("__") !== -1 ?
+            return withoutAtTypePrefix.indexOf(mangledScopedPackageSeparator) !== -1 ?
                 "@" + withoutAtTypePrefix.replace(mangledScopedPackageSeparator, ts.directorySeparator) :
                 withoutAtTypePrefix;
         }