Remove support for dual rebuild-restart for plugin installation

After a series of refactorings no existing plugin requires a two rebuild-restart cycles anymore for successful and complete installation. In v13, support for this can be removed, which will reduce the complexity of the Essentials install state machine significantly.

The situations that previously required dual rebuild-restart have been resolved, we have better practices now, especially adding dependencies and repositories through XML instructions, and copying sample data YAML files into the project files.

We can probably delay support for a repository XML instruction until there is a demand, and we should update the relevant documentation.