Use unit id for package key #2503

Job	Run time
build-extra-hackages	19s
cabal	19s
cabal-doExactConfig	19s
check-materialization-concurrency	26s
check-path-support	9s
closure-size	8s
docs	44s
haskell-nix-roots-do-not-require-IFDs	10s
hix-cabal	15s
hydra-ifdLevel-0-and-1	44s
hix	47s
hydra-ifdLevel-2	31s
hydra-ifdLevel-3-and-ghc-8-10	33s
hydra-ifdLevel-3-and-ghc-9-2	24s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	29s
hydra-without-remote-builders-ghc8107	26s
hydra-without-remote-builders-ghc9101	24s
maintainer-scripts	22s
nix-build	19s
multi-target	14s
nix-tools	19s
plan-extra-hackages	18s
runghc	24s
shellFor-hoogle	23s
shellFor-multiple-package	20s
shellFor-not-depends	17s
shellFor-single-package	17s
tests-benchmarks	16s
unit-tests	15s
update-docs	7s
	10m 58s

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