qmlls: Support module/qmldir imports

https://www.qt.io/blog/compiling-qml-to-c-qtquick-controls-and-optional-imports shows how to perform a module-level/qmldir-level import (use-case: Allow platform-specific Qt Quick Controls styles to be compiled with qmlsc). Unfortunately, these imports are not detected by qmlls in Qt Creator 10.0-beta1.

The following warnings still appear when qmlls is enabled, even after:

1. Building and running the app, to ensure that the qmldir is generated
2. Closing and reopening Qt Creator, to ensure that the code model is fully reset