Old generated signatures persist even after source file is deleted/renamed about spoofax-pie

[Gohla]

Good catch! Currently we do not delete provided files of tasks that become unobserved and are deleted, as a safety measure to not accidentally delete important files. Maybe we can start deleting files that are in the ./build directory, as these are always generated and safe to delete. That would at least show errors after the build completes.

To get errors immediately on rename, we need to get rid of including directories with generated files, and instead directly depend on the tasks that generate those files, with those tasks returning the list of files they've generated. Then, when an SDF3 file is renamed, the old generated signatures are not provided any more, the generator task does not return those files any more, and the import would fail immediately.

Unfortunately, this is not possible with Statix as Statix only supports directory includes at the moment. Statix would have to be changed to support this, which is nontrivial because includes are handled through Stratego primitives during which we do not have access to the PIE context. A similar problem exists for Stratego, but that is probably easier to fix due to the include logic being executed as part of PIE tasks.