Build

Activate more warnings, including `Wunused-packages` for GHC 8.10+ #601

Job	Run time
continuous-integration (8.10.7)	2m 58s
continuous-integration (9.0.2)	2m 38s
continuous-integration (9.2.8)	2m 13s
continuous-integration (9.4.5)	3m 11s
continuous-integration (9.6.4)	12m 12s
continuous-integration (9.8.2)	12m 21s
continuous-integration (9.10.1)	1m 46s
continuous-integration (9.12.1, --allow-newer=base --allow-newer=template-haskell --allow-newer=g...	2m 8s
	39m 27s

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