Discuss pyside2qmlplugin files and folders location

This issue is created in order to discuss how new pyside2qmlplugin library and related tooling (pyside2-qmlscene, pyside2-qmltestrunner, etc.) will affect PySide2 installation folder structure.

1. My thinking is that we should distribute pyside2qmlplugin.so|dylib|dll as a library (subclass of QQmlExtensionPlugin) and in the installation folder put it somewhere like a_virtual_env/lib/python3.8/site-packages/PySide2/Qt/plugins/pyside2qmlplugin folder;
2. Tools like pyside2-qmlscene, pyside2-qmltestrunner should be installed as a_virtual_env/bin/<tool_name>. Those tools will be simple wrappers for Qt/C++ qmlscene, qmltestrunner, with the only difference - they will look for pyside2qmlplugin.so|dylib|dll and load it as a plugin in order to get information about custom python-based QML modules (defined with QmlElement decorator or by subclassing QQmlExtensionPlugin).