nix-generate-from-cpan: large refactor

This commit includes a substantial refactoring of
`nix-generate-from-cpan`. This somewhat simplifies the code through the
use of the CPAN::Meta module while adding the following features:

 - The program now takes an optional maintainer on the command line that
   is subsequently added into the generated package.

 - An attempt is made to convert the license specified inside the
   META.json or META.yaml file to a license in `stdenv.lib.licenses`.

 - An attempt is made to disambiguate attribute names of packages whose
   name is a reserved word in Nix.

 - Write logging output using Log::Log4perl.

 - Print module RSS feed URL. The RSS feed, hosted by MetaCPAN, can be
   used to track updates to the module.