Consider analyzing LLBC directly

In [this Zulip thread](https://aeneas-verif.zulipchat.com/#narrow/channel/568078-Hermes.2FZerocopy/topic/Charon.20--start-from/with/573591699), @nadrieril suggested that we could simplify some extraction by invoking Charon first and then just processing the LLBC. This would make a few things easier:
- LLBC contains non-doc comments, which we currently don't parse – #3031
- This would allow us to support items nested inside function bodies, which we currently reject since we can't name them by path
- We might be able to offload more Cargo resolution logic to `charon`
- #3055
- #3217
- #3026