> I’ll send a new patch once I’ve written a couple of tests
> for all this.

Attached is the latest version of this patch set.  I tried to
incorporate all suggestions and also added tests for the CRAN importer.

I have not implemented the suggested procedure ‘cran-uri’ that generates
URLs for tarballs offered through CRAN, because I couldn’t determine
whether all packages really use the same URL prefix.

~~ Ricardo