Issue title: Concord modules?
 Issue body: We've got a rather large build, and after grep'ing through our codebase, it seems that none of our dependencies are using the concord field in package.json. Is this something that is still being pursued or could it be removed from `enhanced-resolve`?
 Comments: 
 Comment 0: Or at the very least, controllable by an option? I don't know who even uses "concord modules", but it doesn't seem like it's anywhere near the majority.
 Comment 1: It's now controllable by option and off by default.
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