[Go] Package imports itself when using unions with imported members

[tigrato]

Following #5591 a regression, introduced by #5339, was detected when generating Union with imported members.

Version: master - Linux

flatc  --go --gen-object-api --gen-all  --gen-mutable -o ./golang/ Example_fbb.fbs

Example_fbb.fbs:

include "./message/auth_messages.fbs";

namespace Example;

union MsgType {
Auth.ServerAuthReq
}  

table Root  
{  
  message:MsgType;
}

root_type Root;

auth_messages.fbs:

include "./data/common_data.fbs";

namespace Example.Auth;

table ServerAuthReq{
	selfType:Data.ServerType;	
}

common_data.fbs

namespace Example.Data;

enum ServerType : uint8 {
	None = 0,
	DatabaseServer,
	WebServer,
}

The output code in "MsgType.go" imports itself

package Example

import (
        "strconv"

        flatbuffers "github.com/google/flatbuffers/go"
        
        Example "Example"
        Example__Auth "Example/Auth"
)
....

This happens because GenNativeUnion is called before updating cur_name_space_ to the correct enum namespace which eventually is done at GenEnum.

flatbuffers/src/idl_gen_go.cpp

Lines 84 to 90 in 32254b7

	if ((*it)->is_union && parser_.opts.generate_object_based_api) {
	GenNativeUnion(**it, &enumcode);
	GenNativeUnionPack(**it, &enumcode);
	GenNativeUnionUnPack(**it, &enumcode);
	needs_imports = true;
	}
	GenEnum(**it, &enumcode);