[Std] root package

Greetings,

I just noticed that the Haxe STD does Not have a "root package / Namespace", which makes it impossible to exclude the whole std from generating files while building to a target.

Possible use case would be building against the PHP target to Build one Lib which is the STD that gets Autoloaded through Composer and building multiple Lib's with the help of Haxe and using these Libs inside of an Application.
The result currently would be that all of the Libs bring with them their own implementation of the Haxe STD since it gets generated.
But target would be to use the same Haxe STD Implementation among all of them. Reducing the amount of files the server would need to load.

The issue names no files, tests, or compiler entry point. Begin by locating how the Haxe standard library is assigned to generated target namespaces, then define and verify a root namespace that lets PHP builds share one Composer-loaded STD implementation across libraries.

Written by the indexing model from the issue text.