[Conflict] Track the source of every conflicting requirement

Before I merge this, I want to make sure this seems sane?
It enables the following output (the `required by` part) in CocoaPods:
```
[!] Unable to satisfy the following requirements:

- `AFNetworking (~> 1.3.0)` required by `RestKit/Network (0.23.1)`
- `AFNetworking (> 2)` required by `Podfile`
- `RestKit/Network` required by `RestKit/Core (0.23.1)`
- `RestKit/CoreData` required by `RestKit/Core (0.23.1)`
- `RestKit/Core` required by `RestKit (0.23.1)`
```
\c @alloy, @floere