[protobuf] Handle collisions in emitted Protobuf namespaces

Currently, we don't check for name collisions in Protobuf packages, and in some cases, we synthesize names from contextual information. For example, we can create a message for the following operation:

```tsp
op Foo(@field(1) a: int32): void;
```

by representing it with the following Protobuf message:

```proto3
message FooInput {
int32 a = 1;
}
```

This is a nice ergonomic feature, but we aren't currently ensuring that some message named `FooInput` isn't otherwise declared. It should be an error if so.

- [ ] Ensure that the names of messages, services, and enums do not collide within the package.
- [ ] Ensure that the keys of enums also do not collide (enums in Protobuf have C++ scoping).