This is an organizational change from master. It is not strictly
necessary, but backporting it to the release branch will make it
significantly easier to backport patches or updates in the future.
(cherry picked from commit 98d8e1a160)
Also:
- It's now installable by doing "nix-env -i nix-generate-from-cpan".
- It maps dependencies to the correct attribute (e.g. HTML::HeadParser
is mapped to HTMLParser).
- It automatically selects buildPerlPackage or buildPerlModule.
- It's documented in the manual.
