Rewrite the CPAN generator to Perl

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.

.gitignore

@@ -2,6 +2,8 @@
 ,*
 .*.swp
 .*.swo
-cpan-info
-cpan_tmp/
 result
+doc/NEWS.html
+doc/NEWS.txt
+doc/manual.html
+doc/manual.pdf