Add tools to check dependencies

[Pierre Labastie]

There are several kinds of checks we might want to implement, for example:

• (i) check that a package can be built with only the listed dependencies, and their dependencies (missing dependency)
• (ii) check that the dependencies do not include dependencies that are already in the dependency chain (redundant dependency)
• (iii) check that a dependency is really needed (false dependency)

[Pierre Labastie]

Note that (i) is doable (and easy once #1730 is done). I suppose (ii) can be done too, but I have not thought about it yet. For (iii), this is more difficult, because we have to find a criterion that can be automated. Of course, removing all listed deps and use (i) to check, adding deps one by one could be done. But it is quite tedious when the number of deps is big!

[Pierre Labastie]

Implemented (i) in 0a1943912, dedd50263, 42dfc6ef2, 1bf09c15, 4dfe8f99, and 72711ab1. Closing for now: (ii) is not a big problem, and (iii) is really the role of the person updating the package, or occasionally of users.