haxelib run + cwd behaviour

[nadako]

The current behaviour is quite weird: We append cwd to the end of argument list and then change cwd to the library installation directory. What's the reason it's implemented like that? cc @ncannasse

I think it would be more correct not to change cwd and set the lib directory to the HAXELIB_RUN env var (currently it's always set to 1).

Changing this will break current haxelibs, so if we want to do it, we should think of some way to stay compatible.

[nadako]

Assigning to @ncannasse for comments :)

[ncannasse]

I agree with the change. Not sure how to make sure to handle backward compatibility.

[nadako]

One possibility is to add some "haxelib protocol version" field to haxelib.json and use the new mechanism if it's set to proper value (and warn if author submits a library using old mechanism). But I'm not sure it's worth adding that field just for this change.