[display] no completion in <platform>.hx files

[Gama11]

With an empty Main named Main.js.hx, and -js specified, no display modes seem to work:

class Main {
	static function main() {}
}

> haxe -js main.js --display Main.js.hx@0@diagnostics
Error: Display file was not found in class path

Original issue: vshaxe/vshaxe#147

[Yanrishatum]

Regression: Completion no longer provided at least since 4.0.0. Tested on Haxe 4.0.0, 4.0.3 and 4.0.5. (Windows x64)
Output on reproduction sample:

[{"file":null,"diagnostics":[{"kind":2,"severity":1,"range":null,"args":"File \"_build/src/compiler/displayOutput.ml\", line 338, characters 6-12: Assertion failed"}]}]

@Simn