Verify that dev-repo is valid

[rgrinberg]

By doing $ opam source --dev on it. It should be able to catch issues like this:

ocaml/opam-repository#11342

cc @avsm

[AltGr]

Good idea, thanks!

Shouldn't that be part of the normal linting process (which is built-in opam) ?

It'd be useful for local linting, as well. Although you probably don't want it when linting a whole repository...