gostring plugin does not add package name for nullable enums

[godfried]

The gostring plugin does not add the package name of a nullable enum to the generated code.
For nullable enums, a closure is generated so that they can be rendered as pointers. The same approach is used for basic types (e.g. strings, ints).
However, the enums have a package and this is not added to the closure's function signature.
Example

message EnumNullable {
  optional TheEnum TheEnum = 1 [(gogoproto.nullable) = true];
}
enum TheEnum {
	TheEnum0 = 0;
}

GoString result:

&issue640.EnumNullable{TheEnum: func(v TheEnum) *TheEnum { return &v } ( 0 ),
}

Expected:

&issue640.EnumNullable{TheEnum: func(v issue640.TheEnum) *issue640.TheEnum { return &v } ( 0 ),
}