Consider generating enum classes in the same file with messages

Currently classes for enums are defined in separate files. I don't know what the reason was for this, and it's not documented. I asked to a few people and they also don't know. We should just generate enum classes in the same files with messages.

One problem is how to migrate existing code that imports `.pbenum.dart` files without too much disruption. If it's possible to (1) import and re-export things from another file (2) add a deprecation warning to a file, then we can generate a warning when the `.pbenum.dart` files are imported but keep exporting the same classes. Then remove the files in a few releases.

cc @sigurdm