lint: add W74 to detect 'pin-depends' packages that are not present in 'depends' nor in 'depopts'

[rjbou]

fix #5795
This lint doesn't detect when the versions mismatch.

change needed

[kit-ty-kate]

The code can be improved by eliminating 3 out of the 6 datatypes traverses.
The gain is small in this particular case but out of principle it feels weird to not do better if the code is as short and maybe even more understandable

[kit-ty-kate]

Suggested change

	List.map fst t.pin_depends
	|> OpamPackage.Set.of_list
	|> OpamPackage.names_of_packages
	List.fold_left
	(fun (pkg, _) set -> OpamPackage.Name.Set.add (OpamPackage.name pkg) set)
	t.pin_depends OpamPackage.Name.Set.empty

[kit-ty-kate]

Suggested change

	OpamPackage.Name.Set.diff pkgs all_depends
	|> OpamPackage.Name.Set.elements
	|> List.map OpamPackage.Name.to_string
	OpamPackage.Name.Set.fold
	(fun pkgname acc -> OpamPackage.Name.to_string pkgname :: acc)
	(OpamPackage.Name.Set.diff pkgs all_depends) []

[rjbou]

I find the set transformation by pipe more human understandable (you just follow the function name, not read the code of the function given to fold). But agree on the optim part.