Use unit id for package key #2550

Job	Run time
cabal	29s
cabal-doExactConfig	29s
nix-build	1m 4s
runghc	27s
tests-benchmarks	19s
unit-tests	15s
multi-target	54s
shellFor-hoogle	25s
shellFor-multiple-package	30s
shellFor-single-package	36s
build-extra-hackages	21s
docs	52s
hix	42s
hydra-ifdLevel-0-and-1	14s
hydra-ifdLevel-2	22s
maintainer-scripts	33s
plan-extra-hackages	18s
shellFor-not-depends	42s
check-materialization-concurrency	25s
check-path-support	11s
closure-size	11s
haskell-nix-roots-do-not-require-IFDs	11s
hydra-ifdLevel-3-and-ghc-8-10	20s
hydra-ifdLevel-3-and-ghc-9-2	20s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	23s
update-docs	14s
hix-cabal	22s
hydra-without-remote-builders-ghc8107	1m 9s
hydra-without-remote-builders-ghc9101	43s
nix-tools	18s
	14m 19s

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