Use unit id for package key #2476

Job	Run time
cabal	12s
nix-build	27s
runghc	12s
unit-tests	10s
cabal-doExactConfig	10s
maintainer-scripts	10s
multi-target	11s
shellFor-hoogle	12s
shellFor-multiple-package	15s
shellFor-not-depends	11s
shellFor-single-package	15s
tests-benchmarks	10s
build-extra-hackages	13s
check-materialization-concurrency	22s
check-path-support	4s
closure-size	4s
docs	26s
haskell-nix-roots-do-not-require-IFDs	4s
hix	37s
hydra-ifdLevel-0-and-1	31s
hydra-ifdLevel-2	18s
hydra-ifdLevel-3-and-ghc-8-10	21s
hydra-ifdLevel-3-and-ghc-9-2	20s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	20s
plan-extra-hackages	17s
update-docs	6s
hix-cabal	7s
hydra-without-remote-builders-ghc8107	12s
hydra-without-remote-builders-ghc9101	11s
nix-tools	13s
	7m 21s

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