[REG 4.2 -> 4.3] plugins.qbs is missing from project tree

1. Have a checkout of Creator's repository at tag v4.1.0-beta1.
2. Open qtcreator.qbs from this repository.
3. Wait for parsing to finish.
4. Open file src\plugins\plugins.qbs, e.g. using the Locator.
5. In the Projects view, click the "Synchronize with Editor" button.
   Nothing will happen. It seems like the file is not shown in the project tree at all.

The Projects view should list all involved files. Creator 4.2 showed plugins.qbs in Qt Creator/Sources/Plugins

Found by Squish test tst_opencreator_qbs.