Messages with underscores in their name do not generate working python.

Messages with underscores in their name do not generate proper proto.

Consider a message of the following format:

```
message some_message {
enum Type {
type1 = 0;
type2 = 1;
}
Type type = 11;
}
```
The resulting better python output will be:

```
@dataclass(eq=False, repr=False)
class SomeMessage(betterproto.Message):
type: "some_message.Type" = betterproto.enum_field(11)

from . import some_message
```
The import file is non-existent.

This was tested on versions 1.2.5 and 2.0.0b3.