[cmake] qt6_import_qml_plugins without import scanner

i'm not sure if this is feasible on a technical level, but i'm wondering about  qt_import_qml_plugins: it currently parses all qml files at compile time to determine the required qml plugins. this is very convenient (kudos for that), but it becomes a bit of a scalability issue: i'm having a big and granular codebase, where several static libraries bring some functionality including their qml counterpart. so with qt_import_qml_plugins i end up running the import scanner several times during every configure step.

on the other hand it seems that the qml plugins became much more granular with qt-6, so importing them manual via `Q_IMPORT_PLUGINS` becomes quite cumbersome (especially due to differences on different platforms). so i wonder if it may be feasible to have a variation of this cmake function that allows me to specify the "top-level plugins" (like "QtQuick 2.2" or "QtQuick.Layouts 1.2"), which would then import in their dependencies automatically?