Use unit id for package key #2514

Job	Run time
cabal-doExactConfig	4m 17s
multi-target	4m 31s
nix-build	4m 36s
cabal	4m 29s
shellFor-multiple-package	29s
shellFor-single-package	2m 7s
tests-benchmarks	20s
build-extra-hackages	2m 26s
docs	2m 18s
hix	1m 51s
hydra-ifdLevel-0-and-1	3m 29s
hydra-ifdLevel-2	1m 22s
maintainer-scripts	43s
plan-extra-hackages	2m 27s
shellFor-hoogle	26s
unit-tests	2m 19s
check-materialization-concurrency	24s
check-path-support	15s
closure-size	8s
haskell-nix-roots-do-not-require-IFDs	10s
hix-cabal	4m 57s
hydra-ifdLevel-3-and-ghc-8-10	31s
hydra-ifdLevel-3-and-ghc-9-2	24s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	20s
hydra-without-remote-builders-ghc8107	7m 7s
hydra-without-remote-builders-ghc9101	7m 13s
nix-tools	4m 54s
runghc	32s
shellFor-not-depends	24s
update-docs	11s
	1h 5m 40s

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