Custom library file not exist in VirtualFile

Hi, 

I am on a custom language project. I am done with custom project, custom language and custom library. Now I want to use the file in custom library. The problem is, in the code below, there is not any custom library file in virtualFile.

Collection<VirtualFile> virtualFiles =
        FileBasedIndex.getInstance().getContainingFiles(FileTypeIndex.NAME, VerilogFileType.INSTANCE,
                GlobalSearchScope.allScope(project));

However, if I create an Intellij Platform Plugin Project, and add my custom library. The above code works fine, the custom library file would show in virtualFiles. So I don't where the problem is. Is it about Project type, or the custom library?

Some information might be useful:

1, I implemented custom library through this example: https://github.com/intellij-dlanguage/intellij-dlanguage/tree/develop/src/main/java/io/github/intellij/dlanguage/library

2, During adding custom library, it pops out a dialog with title "Choose Categories of Selected Files.", and I choose "Source".

Can anyone give me some hint how to debug this, or an example how to make the custom library file show up in VirtualFile?