qmltc: test plugin interaction

(Splitted from QTBUG-91956)

Test qmltc with plugins: does it know how to handle types from other plugins when they are or arent compiled via qmltc?

Unclear if should be explicitly rejected: if there's a qmldir/qmltypes, the type compiler might actually be able to process the QML types that are plugin-provided (but the generated code is useless)