Use unit id for package key #2498

Job	Run time
cabal	15s
nix-build	15s
runghc	16s
unit-tests	14s
build-extra-hackages	16s
cabal-doExactConfig	15s
hix	35s
docs	33s
maintainer-scripts	16s
multi-target	14s
plan-extra-hackages	15s
shellFor-hoogle	15s
shellFor-multiple-package	15s
shellFor-not-depends	14s
shellFor-single-package	15s
tests-benchmarks	14s
check-materialization-concurrency	23s
check-path-support	7s
closure-size	7s
haskell-nix-roots-do-not-require-IFDs	8s
hix-cabal	11s
hydra-ifdLevel-0-and-1	34s
hydra-ifdLevel-2	20s
hydra-ifdLevel-3-and-ghc-8-10	20s
hydra-ifdLevel-3-and-ghc-9-2	18s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	19s
hydra-without-remote-builders-ghc8107	15s
hydra-without-remote-builders-ghc9101	17s
update-docs	7s
nix-tools	12s
	8m 15s

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