CMake: Install separate plugin for private module

These private sources can not be merged into main plugin, because
a type in qmldir can not be both "internal" and "singleton".

In fact, upstream Qt CMake function would warn you about that, but in
our case it didn't because that Qt function checks for property
QT_QML_SINGLETON_TYPE when sources are being added, not at finalization
call. But here we were setting those properties only after
target_qml_sources(), so Qt didn't complain. Meanwhile our own code
(kind of copy-paste from Qt) generates qmldir at finalize_qml_module
but does not check whether both PRIVATE and QT_QML_SINGLETON_TYPE are
set for a file. So we silently generated wrong metadata without
singletons.