Implement backend for module/import availability

We need to be able to retrieve a list of all modules and their imports.
We need to be able to check if a single import is possible.

Model::possible imports is bad the design for the new code model.

Qt (6) modules/imports can be hard coded.
The project modules/imports (qmldir) have to be dynamic.

We will not cache this anymore.

This is only used in the itemlibrary.