Add a lint error on local packages containing an url field

[kit-ty-kate]

I was checking out the opam file for FStar (https://github.com/FStarLang/FStar/blob/a3be6122b76ec0ca29030e1ff72576dceeede19d/fstar.opam#L35) and noticed they have a url field. This field does not have any purpose and isn't used.

In my opinion opam should raise an error during linting when the url field is present and the opam file is used in "local project mode" (as opposed to "repository mode").

Currently in "repository mode" with --check-upstream, opam lint already raises error 60 when the url is missing when the package isn't flagged as conf, so i think it would make sense to also raise an error when the opposite happens.