before selecting other exits as helpers.
In #40136 we forgot to consider the corner case in #40041, discovered
thanks to analysis#36.
This was causing that sometimes a CC circuit was built when
bwscanner_cc wasn't equal or greater than 1 or the other way around.

We didn't realize about this cause this part of the code is very
confusing. To don't make it even more confusing, i've changed the
internal API:
- `select_helper_candidates`: split funtion into one to select the
  helper candidates, knowing whether to use the relay as exit or not
  and other function `use_relay_as_entry` to decide whether to use the
  relay as entry or not checking CC params.
  Also pass a new arg `relay_as_entry`.
- `create_path_relay`: rewmove not used `cb` arg, add `candidates` arg
  to stop having to select them again later on. Move the no
  `candidates` condition here instead of checking it in
  `pick_ideal_second_hop`.
- `_pick_ideal_second_hope: remove unneded `dest` and `cont arguments,
  rename `is_exit` to `helper_is_exit`. Use the candidates instead of
  selecting them again.
- `measure_relay`: in the case an exit fails to exit, select the
  candidates knowing that they have to be exits and checking CC.
- `only_relays_with_bandwidth`: remove unneded arg `controller` so
  that there is no need to pass it through several functions.

Closes #40138