Check that the dune constraint matches the dune-project file

[kit-ty-kate]

Currently opam-dune-lint does not check for the validity of the dune constraint however this is a desirable feature that would make adding an opam-dune-lint phase to opam-repo-ci desirable in the future (we might want to think about the implications of having every explicit dependencies be listed, first)

Implemented in opam-repo-ci in ocurrent/opam-repo-ci#90. Feel free to copy/paste what's needed.

[moyodiallo]

This is fixed, thanks for the indication. Your implementation was very useful, with a slight modification it was OK.