Supporting 'plugin' in directory listing 'qmldir' files

Hi there, we got a "private importing" problem with QML in our project.

- qml // global import path
- plugins
| - Plugin A
  | - main.qml // import "./Private"
  | - Private // invisible for Plugin B 
    | - plugin.dll
    | - qmldir
| - Plugin B
...

In our plug-in based project, some of the modules are required to be imported privately and don't want to give any accesses for any other plug-ins. To import locale QML and JS files privately, we can use the "Directory Listing qmldir Files" infrastructure.

However, the 'plugin' syntax is not supported in a directory listing qmldir file, so it doesn't work for private C++ plug-ins.

It would be nice to have plugin syntax support in directory listing qmldir files in such case.