Fix Enum with module in generated TS interface.

[sampajano]

Verified with the example in #1271

// enum.proto

syntax = "proto3";

enum DocEnum {
    DOC_ENUM_PDF = 0;
    DOC_ENUM_HTML = 1;
}

// test.proto

syntax = "proto3";

import "enum.proto";

package Test;

message HelloRequest {
    DocEnum doc = 1;
}

Generate:

protoc -I=. *.proto --js_out=import_style=commonjs,binary:. --grpc-web_out=import_style=typescript,mode=grpcwebtext:.

Before

getDoc(): enum_pbDocEnum;
setDoc(value: enum_pbDocEnum): HelloRequest;

After

getDoc(): enum_pb.DocEnum;
setDoc(value: enum_pb.DocEnum): HelloRequest;

---

Fixes #1271

[stanley-cheung]

LGTM. Thanks!