CMake: Fix auto-linking of static plugins for non-QML in-tree tests

Certain repositories like qtsvg, qtimageformats and qtvirtualkyboard
build plugins associated with Qt modules from other repositories
(qtsvg's QSvgPlugin associated to qtbase's QtGui).

When configuring in-tree tests in the same build folder as the
repository, the test executables would not automatically link to these
plugins.

Fix this by recording the existence of such plugins in a separate
property of the associated Qt module and only link them when both the
test executable and plugin are from the same project (their
PROJECT_NAME coincides).
This is in addition to linking the plugins associated with the
module where both are built in the same repository.

The logic is a bit tricky and ensures that plugins are not
accidentally initialized twice, so that in-tree tests work for both
top-level and per-repo builds.

As a drive-by, added a TODO explaining why in-tree tests that need to
link to static QML plugins won't work (somewhat unrelated to this
change).

Amends 734d2cdbc4ff6db6b3df8fffbb23dbbb565c076b
Amends b1fcdad9c9b9ad2bddd00f7301c8dd1159d523c2

Pick-to: 6.1
Task-number: QTBUG-87580
Change-Id: I3e1ff8166864f92dea931ec2ea34b6f56b4eec60
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

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