add deps between extra tasks

It may happen that tasks listed in BBTRACER_EXTRATASKS are executed in parallel, leading to race conditions in reading/writing to fileindex. The issue cannot be managed by lockfiles, because lockfiles can lock only functions, not tasks, and we need to keep the lock on fileindex from bbtracer_prefunc to bbtracer_postfunc. Instead of using lockfiles, we add dependencies between extra tasks, so that they are not executed in parallel. In doing that, we must not create duplicate or recursive dependencies.