Use unit id for package key #2472

Job	Run time
build-extra-hackages	21s
cabal-doExactConfig	21s
cabal	24s
check-materialization-concurrency	24s
check-path-support	10s
closure-size	9s
docs	53s
haskell-nix-roots-do-not-require-IFDs	12s
hix	50s
hix-cabal	12s
hydra-ifdLevel-0-and-1	49s
hydra-ifdLevel-2	23s
hydra-ifdLevel-3-and-ghc-8-10	21s
hydra-ifdLevel-3-and-ghc-9-2	18s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	24s
hydra-without-remote-builders-ghc8107	11s
hydra-without-remote-builders-ghc9101	9s
maintainer-scripts	14s
multi-target	12s
nix-build	14s
nix-tools	10s
runghc	13s
plan-extra-hackages	12s
shellFor-hoogle	13s
shellFor-multiple-package	12s
shellFor-not-depends	14s
shellFor-single-package	19s
tests-benchmarks	13s
unit-tests	10s
update-docs	6s
	9m 13s

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