For locally pinned & local dev packages, use any `opam` file located at the root of the path as overlay.

We do that only for local packages, because 1/ it's more complex (need to download the package to check if there is an OPAM file at the root) and 2/ we still want to be fix any wrong OPAM file by modifying the overlay (and if the default is to overwrite the overlay, we completely loose that power).