Add META.json and CPAN::Meta v2 support

This commit does the following:

- adds v2 required fields to the dist object
- populates 'prereqs' from old 'requires' if provided
- generates META.yml and META.json via CPAN::Meta
- generates Makefile.PL from v2 'prereqs' data
- tests the above
rejigger building of v2-style prereqs