Overload the `get` function of `Requests` so that, in addition to the
usual `Mapping` behavior, will return an empty `ObjectSet` if no default
is specified.
Improve the `schedule` in the `Pipeline` class by introducing two
optimizations:
* Prune all the `ContainerDeclarations` that are not actually used.
* Skip all tasks that have no outgoing elements that would have been
written by them.
Now the pype command has a `--verbose` argument that to enable debug
logging in the pypelien code.
Adapted the codebase to use this new logging format but replacing
`logging` with `revng.pypeline.utils.logger`, this is done in
preparation of debug-log.
Giacomo Vercesi