CMake: Export 3rdparty dependency find_package calls of private modules

Consider a Qt module with a 3rdparty library target in
PRIVATE_MODULE_INTERFACE, e.g. XKB::XKB in Qt6::GuiPrivate. Consumers of
GuiPrivate automatically depend on XKB::XKB. In order to do that they
must find_package(XKB ...). As all find_package calls for GuiPrivate are
in the same place as the ones for Gui, this package must be marked as
optional. Otherwise all consumers of Qt6::Gui would have to have the
xkbcommon package installed too.

This patch exports find_package calls for every 3rdparty public
dependency of private modules and marks them as optional.

Change-Id: Ia1eeb09c29927fb6634ef08b477684ed6f123267
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

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