The pro2cmake.py conversion script faithfully reproduced the .pro files
for the plugins, which specified the libraries as public. But in CMake,
the implications of this are that public usage requirements should then
be propagated to consumers. We don't expect any consumers, since a
plugin is created as a MODULE library in CMake, so for Windows we don't
even have an import library to link with. The only exception to this is
for static builds where plugins are created as STATIC libraries
instead, but only in certain controlled situations do we then link to
plugins. Even then, usage requirements are not expected to propagate to
the consumers, so these relationships should always be specified as
private.

Task-number: QTBUG-90819
Change-Id: I97ac6b81cfc89ded50e9f4038f53c8c7c98c3f4c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit 864ae072

)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>