Support multiple packages with deps that have the same versions

I mis-configured wit-bingen macro and got the error message:

```
proc macro panicked
message: assertion failed: prev.is_none()
```

When trying to debug it I tried using `cargo expand` and `cargo docs` to track down what I did wrong but the gave the same error.

After some digging and forking the various repositories, I eventually tracked the error message to this repo which has a bunch of asserts: https://github.com/search?q=repo%3Abytecodealliance%2Fwasm-tools%20%20prev.is_none&type=code

As a developer it would be nice to have a better error message for these asserts to improve the debugging experience.