Add linting check for new restricted prefixes

[shonfeder]

Following ocaml/opam-repository#25861, the opam repository aims to treat the following package name prefixes as restricted:

• arch-
• system-
• host-

The need linting rules added to the CI checks on opam files to raise an alert when package names start with one of these prefixes.

cc @raphael-proust and @kit-ty-kate

[benmandrew]

Another possible linting check that's being asked for is to check that a package has one of these prefixes iff it also has the corresponding conflict-class field.