* guix/import/cpan.scm (module->dist-name): Fetch the field of interest.
(cpan-fetch): Accept release name rather than module name.
(fix-source-url): Rename to ...
(cpan-source-url): ... this. Take metadata as parameter.
(package->upstream-name, cpan-version, cpan-package?, latest-release):
New procedures.
(cpan-module->sexp): Use cpan-version and cpan-source-url.
(%cpan-updater): New variable.
* guix/scripts/refresh.scm (%updaters): Add %CPAN-UPDATER.