Message names with underscores and without result in two dataclasses with the same name

If you have a message called, `some_message` and `SomeMessage`, the resulting python has two dataclasses called `SomeMessage`.

Start by reproducing the issue with protobuf messages named `some_message` and `SomeMessage`, then inspect the generated Python to find where message names are converted to dataclass names. Done means the generated output no longer defines two dataclasses with the same name, with a regression test covering both message names.

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