guix: opam: More flexibility in the importer.

guix: opam: More flexibility in the importer.

* guix/scripts/import/opam.scm: Pass all instances of --repo as a list
  to the importer.
* guix/import/opam.scm (opam-fetch): Stop expecting "expanded"
  repositories and call get-opam-repository instead to keep values
  "symbolic" as long as possible and factorize.
  (get-opam-repository): Use the same repository source as CLI opam does
  (i.e. HTTP-served index.tar.gz instead of git repositories).
  (find-latest-version): Be more flexible on the repositories structure
  instead of expecting packages/PACKAGE-NAME/PACKAGE-NAME.VERSION/.
* tests/opam.scm: Update the call to opam->guix-package since repo is
  now expected to be a list and remove the mocked get-opam-repository
  deprecated by the support for local folders by the actual
  implementation.
* doc/guix.texi: Document the new semantics and valid arguments for the
  --repo option.

Signed-off-by: Julien Lepiller <julien@lepiller.eu>