A bear base class that parallelizes file-wise.

A ProjectBear runs on all files at once, so it has the worst degree of
parallelization. It just spawns a single task.

... when scheduling all dependency bears. Bears that offload no tasks
may cause the event-loop to close too early while other bears had no
chance to submit their tasks.

Fixes #4505

Allows to submit tasks on a per-dependency-result-basis.

Closes #4425