Linting for package source is not checking mirrors

[shonfeder]

In ocaml/opam-repository#27987, mirrors are added to the important ocamlfind packages. However, the linting check is not checking the mirror and we are not actually getting to running the checks.

In addition to fixing the linting logic, this will also require fixing the current pipeline.

https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/883026b00b1de517379e39378fb6bb6301cf0a3c