Improve depext unavailable messages from the solver

[dra27]

Well, hopefully improve. At the moment, if you install a conf package directly, you get the depext availability message from before solving:

dra@thor:~/opam$ opam install conf-capnproto
[ERROR] Package conf-capnproto depends on the unavailable system package
        'capnproto libcapnp-dev'. You can use `--no-depexts' to attempt
        installation anyway.

although the message is not properly formatted. If, as is more likely, the conf package is pulled in via a dependency, the message comes after solving and is a little more intimidating:

dra@thor:~/opam$ opam install capnp-rpc-unix
[ERROR] Package conflict!
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-lwt < 0.2 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-lwt >= 0.2 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net < 0.6.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net = 0.6.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net = 0.7.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net = 0.8.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net = 0.9.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net = 1.0 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.
  * Missing dependency:
    - capnp-rpc-unix → capnp-rpc-net >= 1.1 → conf-capnproto
    depends on the unavailable system package 'capnproto libcapnp-dev'. Use
      `--assume-depexts' to attempt installation anyway.

No solution found, exiting

(and also features the same formatting problem).

This PR fixes the formatting of multiple missing depexts, so you now get:

dra@thor:~/opam$ opam install conf-capnproto
[ERROR] Package conf-capnproto depends on the unavailable system packages
        'capnproto' and 'libcapnp-dev'. You can use `--no-depexts' to attempt
        installation anyway.

and also spots that all the chains refer to the same missing dependency and reduces that to one explanation. It doesn't display any chain with that, because I don't think it's useful - all possible solutions required this depext, so the package name should be obvious to the user. Conversely, picking the right chain to display is extremely tricky, if even possible. You now get the slightly less scary-looking:

dra@thor:~/opam$ opam install capnp-rpc-unix
[ERROR] Package conflict!
  * Missing dependency:
    - conf-capnproto
    depends on the unavailable system packages 'capnproto' and
      'libcapnp-dev'. Use `--assume-depexts' to attempt installation anyway.

No solution found, exiting

The reduction should be extended to group the explanations first, but I wanted to get some feedback on the refactoring before doing that too.

[rjbou]

LGTM!