Support OR clauses in depexts field

[raphael-proust]

Some external dependencies may be available as either of a set of packages from the distribution. E.g., rust is available under Archlinux when installing either rust or rustup. Currently conf-rust checks for the rust package. It would be useful if we could specify

 ["rust" | "rustup" ] {os-distribution = "arch"}

as a depext. This does not seem to be possible (according to the documentation).

Are there issues I haven't considered?

(This issue is possibly related to #3140 in that there is overlap in the concerns.)

[dra27]

This would be expressible, if not terribly elegantly, by having conf-rust and conf-rustup and depending on (("conf-rust" | "conf-rustup"). (cf. ocaml/opam-repository#19338)

[dra27]

(doesn't rule out the feature being useful as well, though!)