Convert packages from opam to cudf directly

We used to convert through Debian, which had its merits, but their needs differ.
Doing the conversion directly actually yields simpler code, reduces the potential bug surface,
makes the the cudf files smaller AND gives us more expressivity.

Closes #1185
Closes #1179

Instead of sorting all _mentionned_ versions of a package as done in Debian and mentionned by https://github.com/ocaml/opam/issues/1179#issuecomment-35426135
I chose to sort all _existing_ versions. This has the merit of being much more stable, as we can have universes with the same packages but different constraint sets (for optional dependency handling). This used to be handled with a hack that made sure to still _mention_ the same packages.
Of course, constraints need then to be rewritten if they were mentionning versions that didn't exist, but that's not too complex.

We still rely on the Debian _version ordering_ function, there should be no harm in that.
Skip the final 'command opam xxx failed' unless in verbose mode

this is redundant since there must be more precise error messages, and we already
got the return value of the program for that
Cause computation: factorise a tiny bit of code
Fixed resolution with the internal heuristic

'dose-dummy-request' packages were left in the universe.
Don't download archives in 'fake' mode
Improve action debug printing
Make the --cudf option also output a graph of the actions