Move haxelib to higher level

[Simn]

Almost forgot about this branch. This moves haxelib handling from the low-level args parsing to the high-level process_params. I also took the opportunity to incorporate some of the changes from #10393.

In theory this should now allow us to provide a different compiler entrypoint.

[Simn]

Oh yeah, I had to add the --neko-lib CLI argument. I realize that this seems pretty silly in 2022, but we have to communicate the special -L handling from haxelib to the lower level somehow. I have no idea if this is actually still used by some haxelibs, but for now I didn't want to risk it.