Passing INTERNAL_MODULE to qt_internal_add_module should only create a single Qt::FooPrivate target

Resulted from discussion at https://codereview.qt-project.org/c/qt/qtbase/+/318453/2/src/entrypoint/CMakeLists.txt#12

With qmake, marking a qt module with CONFIG += internal_module ends up creating a single qt_lib_foo_private.pri file, without a qt_lib_foo.pri counterpart.

In CMake, marking a module as INTERNAL_MODULE ends up creating 2 targets: Qt::Foo and Qt::FooPrivate.
It's currently done this way because we realised the required changes too late in the CMake port timeframe, and it was easier to place all relevant include dirs, etc on the main non-Private target.

We should align with qmake, and create only the Qt::FooPrivate target.