Header only Qt modules shouldn't propagate Qt::PlatformModuleInternal as public dependency

Investigated from https://bugreports.qt.io/browse/QTCREATORBUG-25283

qt_internal_add_module unconditionally adds a private Qt::PlatformModuleInternal dependency on the created Qt module.

However if the module is a HEADER-only module (like QtUiPlugin in qttools), we automatically move all dependencies to be added to INTERFACE_LINK_LIBRARIES, because we can't set LINK_LIBRARIES for a header only module.

This causes all consumers of the header module to bring in compilation flags of Qt::PlatformModuleInternal, which is not what we want for public user projects.

In the linked issue a Qt Creator target links to the Qt::Designer lib from qttools, which brings QtUiPlugin -> Qt::PlatformModuleInternal dependency, that causes Creator code to compile with the -fapplication-extensions flag and that causes issues in the context of PCH on macOS.

Investigate if the solution is merely to remove the PlatformModuleInternal public dependency for header only modules, or if it requires something more.