Use unit id for package key #2477

Job	Run time
cabal-doExactConfig	3m 5s
check-materialization-concurrency	26s
build-extra-hackages	3m 3s
cabal	3m 7s
check-path-support	1m 57s
closure-size	9s
haskell-nix-roots-do-not-require-IFDs	1m 4s
docs	1m 28s
hix	56s
hix-cabal	17s
hydra-ifdLevel-0-and-1	47s
hydra-ifdLevel-3-and-ghc-8-10	25s
hydra-ifdLevel-2	27s
hydra-ifdLevel-3-and-ghc-9-2	17s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	19s
hydra-without-remote-builders-ghc9101	27s
hydra-without-remote-builders-ghc8107	30s
maintainer-scripts	12s
nix-build	23s
multi-target	24s
nix-tools	17s
plan-extra-hackages	16s
runghc	20s
shellFor-hoogle	26s
shellFor-multiple-package	28s
shellFor-not-depends	31s
shellFor-single-package	32s
tests-benchmarks	32s
unit-tests	26s
update-docs	12s
	23m 43s

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