* guix/import/cran.scm (%bioconductor-svn-url): Remove variable.
(bioconductor-mirror-url): New procedure.
(fetch-description): Take a REPOSITORY symbol instead of a BASE-URL string.
(cran->guix-package): Pass REPOSITORY symbol to "fetch-description".
(latest-cran-release, latest-bioconductor-release): Adjust accordingly.
(bioconductor-package?): Update comment about SVN.