* guix/import/cran.scm (download): Handle the case where URL is a list.
(fetch-description-from-tarball): New procedure.
(fetch-description): Add #:version parameter. Honor it when REPOSITORY
is 'cran. Use 'fetch-description-from-tarball' when REPOSITORY is
'bioconductor.
(description->package): SOURCE-URL may now be a list.
(cran->guix-package): Pass VERSION to 'fetch-description'.
(cran-recursive-import): Add #:version parameter.
* guix/scripts/import/cran.scm (guix-import-cran): Expect a spec rather
than a mere package name.
* doc/guix.texi (Invoking guix import): Document it.
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