[dce] Enum is not eliminated if placed in a separate module and referenced through a getter

Main.hx
```haxe
import TestClass;

class Main {
static function main() {
}
}
```
TestClass.hx
```haxe
class TestClass {
public var stdio(get,never):Array;
function get_stdio():Array return null;
}
```
TestEnum.hx
```haxe
enum TestEnum {}
```
Generated js:
```js
// Generated by Haxe 4.1.0-rc.1
(function ($global) { "use strict";
var $_;
function $extend(from, fields) {
// <...>
}
var Main = function() { };
Main.__name__ = true;
Main.main = function() {
};
Math.__name__ = true;
var TestEnum = $hxEnums["TestEnum"] = { __ename__ : true, __constructs__ : [] // <--- here
};
// <...>
```
As you can see `TestEnum` is not used, but still kept.
That happens for any target, but on js it also causes an exception on startup:
```
bin/test.js:15
var TestEnum = $hxEnums["TestEnum"] = { __ename__ : true, __constructs__ : []
^
ReferenceError: $hxEnums is not defined
at /home/alex/temp/test/bin/test.js:15:16
at Object. (/home/alex/temp/test/bin/test.js:100:3)
<...>
```
Changing `stdio` getter from `get` to `default` fixes it.
This is reproducible with 4.0.5 and the latest dev.