Use unit id for package key #2504

Job	Run time
nix-build	5m 32s
runghc	4m 18s
cabal	5m 34s
cabal-doExactConfig	5m 33s
shellFor-multiple-package	40s
multi-target	21s
shellFor-single-package	1m 7s
tests-benchmarks	20s
unit-tests	14s
build-extra-hackages	22s
docs	8m 24s
hix	1m 42s
hydra-ifdLevel-0-and-1	1m 52s
hydra-ifdLevel-2	1m 18s
hydra-ifdLevel-3-and-ghc-8-10	18s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	18s
maintainer-scripts	1m 41s
plan-extra-hackages	17s
shellFor-hoogle	12s
shellFor-not-depends	13s
check-materialization-concurrency	27s
check-path-support	1m 13s
closure-size	7s
haskell-nix-roots-do-not-require-IFDs	1m 15s
hix-cabal	3m 12s
hydra-ifdLevel-3-and-ghc-9-2	18s
hydra-without-remote-builders-ghc8107	7m 6s
hydra-without-remote-builders-ghc9101	9m 8s
nix-tools	5m 31s
update-docs	8s
	1h 8m 41s

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