* guix/import/cpan.scm (<cpan-dependency>, <cpan-release>): New
JSON-mapped record types.
(metacpan-url->mirror-url): New procedure.
(cpan-source-url): Rewrite in terms of it.
(cpan-version): Remove.
(cpan-module->sexp): Rewrite to take a <cpan-release> instead of an
alist, and rename 'meta' to 'release'.
[convert-inputs]: Rewrite to use 'cpan-release-dependencies'.
Update calls to 'convert-inputs' to pass a list of symbols.
Replace 'assoc-ref' calls with the appropriate field accessors.
(cpan->guix-package): Rename 'module-meta' to 'release'.
(latest-release): Likewise, and use the appropriate accessors.
* tests/cpan.scm (test-json): Remove "prereqs" record; add "dependency"
list.
("source-url-http", "source-url-https"): Remove.
("metacpan-url->mirror-url, http")
("metacpan-url->mirror-url, https"): New tests.
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