Nixpkgs convention is to only add attributes for deprecation throws
when config.allowAliases hasn't been explicitly disabled. This is
useful as it makes it easy to test building all non-deprecated
packages:
nix-build --arg config '{ allowAliases = false; }' -A sourcehut
Precedent for this convention in Nixpkgs is easy to find by grepping
for "config.allowAliases".
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