CMake: Postpone target existence check for qml plugin targets

Each included qml Qt6FooPluginTargets.cmake file checks whether
all the dependency targets that are referenced in the file already
exist by the time the file is included.

If one of the referenced targets is missing, the file sets
${CMAKE_FIND_PACKAGE_NAME}_NOT_FOUND_MESSAGE with a message mentioning
which targets are missing and also sets
${CMAKE_FIND_PACKAGE_NAME}_FOUND to FALSE.

All our qml package Config.cmake and Targets.cmake files are
include()d by Qt6QmlPlugins.cmake using a file(GLOB) which means the
order in which the files are loaded is implementation-defined.

Furthermore we didn't check the above set variables after each
inclusion, which means the values are overridden and the last plugin
to be loaded determines whether the Qml package is found or not.
If the last included file sets no error, it effectively silences any
previously set error.

Ever since we added dependencies between the qml plugin targets
themselves, we hit the above situation and depending on the platform,
no error was shown because the last file overrode any errors.

But we finally got a specific platform (wasm) which unearthed the
problem (QTBUG-97478). This can happen for any static Qt build though.

To fix this properly, we will most likely have to rewrite the whole
inclusion mechanism to use find_package() so that dependencies
can be resolved recursively as needed. This is a non-trivial change
that will have to be addressed in both qtbase and qtdeclarative.

In the mean time, a stop-gap solution that this change implements is
to include all the files while ignoring any error messages.
Then include the files one more time and check for error message after
each included file.

All qml plugin targets should have been brought into scope with the
first round of inclusions, thus circumventing "missing referenced
target" errors, while still catching any other possible errors.

Amends
6fd1216801f078f65be2cbc748cc459cb6912a4f
9fc302e6d146878103b3d105dce49c7695fcf93a
c368175a9e0a0c120b5bb8a0a02859bfc0cf42ba
in qtdeclarative.

Pick-to: 6.2 6.2.1
Fixes: QTBUG-97478
Task-number: QTBUG-95609
Task-number: QTBUG-97099
Change-Id: I157fa93fc979d726cd221d969b995b3642aeec77
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

0 files changed, 0 insertions, 0 deletions