HashLink vs Neko

Just a few things we might want to discuss/decide before releasing Haxe 4.0 final.
I'm planning to have HashLink replace Neko in the long term, so we should maybe start doing a few things in that direction:

* [x] `haxe -x` should use eval target instead of neko one
* [ ] `haxelib run` should check for `Run.hx` and `run.hl` and execute either with eval or hashlink
* [ ] try if `haxelib` can be run using eval instead of Neko
* [ ] open `haxelib` question : can we find a way to tag libraries as requiring haxe 4 so that they don't get downloaded by people still on haxe 3 (they would get the oldest version not requiring haxe 4)
* ~we should install HashLink with 4.0 installer additionally to Neko~

These are open to discussion, in case I overlooked something.