Single package fixpoint step 2: introduce `mkPackage` #42476

Job	Run time
get-merge-commit / resolve-merge-commit	4s
Attributes	1m 13s
Eval nixpkgs with aliases enabled	46s
Outpaths (aarch64-linux)	3m 36s
Outpaths (aarch64-darwin)	3m 27s
Outpaths (x86_64-linux)	3m 48s
Outpaths (x86_64-darwin)	3m 26s
Process	0s
Tag	0s
	16m 20s

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