shiboken generates bad flags names

When shiboken generated the names of a flags structure, it used the plain name
in the type name field.

When we turned to using the limited API, all types became heap types via the
new type creation API, that does not allow plain names. A dot must be inserted
to parse some module name off.
The used prefix was arbitrarily chosen as "PySide2.libpyside.".

When now support for type hints was developed, everything worked fine but
flags names.
Example: We got "flags: PySide2.libpyside.ProcessEventsFlags" back, although it
should have been "flags: PySide2.QtCore.QEventLoop.ProcessEventsFlags".