* guix/import/cran.scm (package->upstream-name): Return #f if url
start and end index could not be determined.
(cran-package?): Check if the upstream-name can be extracted from
given package.
* tests/cran.scm: Add "r-minimal is not a cran package" to make sure that
r-minimal is not detected as a cran package.
This fixes a failure of guix refresh on r-minimal because no
upsteam-name can be determined from ".../R-version.tar.gz" uri.
