Abstract class name clash error

If you have in Test.hx
```haxe
abstract Test(_Test) {
public static var a = 2;
}
class _Test {}
```
You'll get the error message:
```
Test.hx:1: lines 1-4 : This private class name will clash with _Test
```
which isn't very explicit.

Also doesn't happen if the abstract isn't the main type of the module.