This mechanism wasn't actually used for anything other than a minor
external compatibility thing for buildGo.nix, which can also be solved
with a function parameter.
This breaks //fun/gemma because it means that the elmPackages used to
build it are no longer reachable from depot. We'll sort this out later.
Change-Id: I1bf2240435e869cdc4e99bdd1a138fdd2e76f96e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2646
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
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