ENH: Add support for softlink inside eggs.

Framework builds of Qt on darwin rely on softlinks, so I thought it would be a good occasion to add softlink support in eggs.

If this change is deemed too dangerous, I could workaround it by creating softlink at post install time but I would rather minimize post install operations as much as possible.

@rkern would be great if you could have a brief look at it, if only to give your opinion on adding support for softlink in eggs, thanks.