CMake: Allow opting out of global target promotion

The endless saga of fighting with qt_find_package and global target
promotion. In certain scenarios we want to opt out of target promotion
to global scope.

One such case is in qttools with WrapLibClang and Threads::Threads.
Threads::Threads will be found in the top-level scope via
Qt6Dependencies.
WrapLibClang is declared in src/ directory scope, and
then we try to promote it ands its dependencies to the global scope,
via qt_find_package() ->
qt_find_package_promote_targets_to_global_scope().

This fails because we can't promote Threads::Threads due to it being
added in a different subdirectory scope.

Introduce 2 new functions.
qt_internal_should_not_promote_package_target_to_global and
qt_internal_disable_find_package_global_promotion.

The first one is used to disable promotion of targets to global scope
in qt_find_package.

To mark a target not to be promoted, the second function is used.
It will be used by qttools for the WrapLibClang case.

Task-number: QTBUG-85877
Change-Id: If6caf10a94999402026517a623ae29e3ab1eeb7f
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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