import: cpan: Adapt for the change to guile-json version 3.

import: cpan: Adapt for the change to guile-json version 3.

In guile-json version 3, JSON objects are represented as hash tables, rather
than alists.

* guix/import/cpan.scm (string->license): Change the match expression to match
on lists, rather than vectors.
(module->dist-name, cpan-source-url, cpan-version): Change assoc-ref to
hash-ref.
(cpan-module->sexp): Change assoc-ref to hash-ref, and assoc-ref* to
hash-ref*.
* tests/cpan.scm ("source-url-http", "source-url-https"): Convert the alist to
a hash table.