import: Add Bioconductor importer and updater.

* guix/import/cran.scm (%bioconductor-updater,
latest-bioconductor-release, bioconductor-package?): New procedures.
(cran->guix-package): Support repositories other than CRAN.
(%bioconductor-url, %bioconductor-svn-url): New variables.
(description->package): Update signature to distinguish between packages
from different repositories.
(latest-release): Rename procedure ...
(latest-cran-release): ... to this.
(cran-package?): Do not assume all R packages are available on CRAN.
* tests/cran.scm: Update tests.
* guix/scripts/import/cran.scm: Add "--archive" option and default to
CRAN.
* guix/scripts/refresh.scm (%updaters): Add "%bioconductor-updater".
* doc/guix.texi: Document Bioconductor importer and updater.