Since we now have a versioned configuration-ghc-*.nix file for GHC HEAD,
we don't need to add a super special case to the package set logic in
test-configurations.nix anymore. We can just create a versioned
attribute for the ghcHEAD package set (which is not exposed) and keep
using the normal discovery logic.
The only tricky bit is that GHC HEAD's configuration file is named after
the GHC release that will be branched off from it, so a little bit of
arithmetic is involved.
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