Use CPAN src index directly

For finding perl tarballs or listing available Perl's, we can
look directly at http://www.cpan.org/src/5.0/ rather than
rely on CPAN::Perl::Releases or scraping search.cpan.org.

This commit also adds an "--all" flag for "available" to list every perl
tarball available.
Merge pull request #331 from dagolden/topic/use-cpan-src-index-directly

Use CPAN src index directly