"Simplify" versionless targets

Versionless targets in Qt6 are interface libraries that link the
versioned libraries using the INTERFACE link type. This makes the
linking chain more complicated than it can be. Also we miss some
significant interface properties in the versionless targets comparing
to the versioned targets.

The new approach manually generates the versionless targets, instead
of using CMake exports.

For CMake versions < 3.18 we now create a copy of the versioned
targets. The copy includes all the relevant INTERFACE properties from
the versioned targets and imported locations for all configs.

For CMake versions >= 3.18 we now create the versionless target ALIASes
which should behave give the transparent access to the versioned
targets.

Using the QT_USE_OLD_VERSION_LESS_TARGETS flag you may force the
behavor of the CMake versions <= 3.18

The change is partial workaround for QTBUG-86533.

Task-number: QTBUG-114706
Change-Id: Iafadf6154eb4912df0697648c031fcc1cbde04e0
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

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