File system hygiene

[dra27]

See ocaml/opam-repository#14288 - it'd be helpful if Camelus warned about unexpected files in a pull request. The valid patterns would be:

• Root files should probably a generate a highlighted note
• /packages/FOO/BAR should be rejected, unless BAR is a directory or a .gitattributes file (the latter should probably generate a highlighted note too)
• /packages/FOO/FOO.VERSION/BAR should be rejected, unless BAR is opam

[thomasblanc]

I'm currently implementing it (see ocaml/opam-repository#14626 for my first attempt, which prints every non-opam file). Yes, I know, I need to use proper markup formatting.

[dra27]

👍