[Do not merge*] Restore stdenv debugging at the cost of more complexity #20889
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#15043 simplifies and cleans up the way stdenvs are bootstrapped, at the cost of removing potentially useful debugging aids. This adds back debugging aids at least as useful as the old ones, but at the cost of some complexity (though the extra stuff should all be dead code when not debugging.)
I'm, not sure whether this is a good idea or not--my hope is that by opening this anyone who's workflows are gummed up by #15043 has somewhere to turn. If that's you, feel free to merge.