Reduce opam-repository maintainer toil by checking depends stanza in opam files

[mbacarella]

Would be nice if opam-publish validated the depends stanza a bit. I published a package recently where I had completely invalid version numbers, and also unwittingly had a pinned package as a direct dependency, and it created a bit of back-and-forth with the opam repository maintainers. Perhaps could have been avoided if opam-publish did some work on the client side first, before pushing.

PR with my proposed fix forthcoming.

[mbacarella]

@camelus @kit-ty-kate this is relevant to your interests :)