Namespaced enums yield incorrectly generated code

Given following proto file

```
syntax = "proto3";
package namespaced_enum;

message ns { enum Type {
NONE = 0;
option_one = 1;
}}

message Msg {
ns.Type t = 1;
}
```

The command `python -m grpc_tools.protoc -I . --python_betterproto_out=lib ./test.proto` generates following code:

```py
# Generated by the protocol buffer compiler. DO NOT EDIT!
# sources: proto/test.proto
# plugin: python-betterproto
from dataclasses import dataclass

import betterproto

from .namespaced_enum import ns

class NsType(betterproto.Enum):
NONE = 0
option_one = 1

@dataclass
class Ns(betterproto.Message):
pass

@dataclass
class Msg(betterproto.Message):
t: ns.Type = betterproto.enum_field(1)
```

It contains line `from .namespaced_enum import ns` and later on uses this as type `t: ns.Type = betterproto.enum_field(1)` instead of `NsType`

---
Python 3.8.2
betterproto 1.2.5
grpcio_tools 1.34.1