import: cran: Support experiment and annotation packages.

import: cran: Support experiment and annotation packages.

* guix/import/cran.scm (%bioconductor-packages-list-url): Replace variable...
(bioconductor-packages-list-url): ...with this procedure.
(bioconductor-packages-list): Accept optional TYPE argument.
(latest-bioconductor-package-version): Same.
(fetch-description): Determine package type and use it in calls to
LATEST-BIOCONDUCTOR-PACKAGE-VERSION and BIOCONDUCTOR-URI.
(description->package): Pass package type to URI helper procedure; include
package type in annotation or experiment packages from Bioconducter.