Add support for the cpanfile "mirror" values

[zebardy]

This patch should provide support for the cpanfile "mirror" values. The mirror values will be superseded by any mirror values that are set as command line parameters, but will be a higher priority than the default mirror value.

[miyagawa]

Thanks for the patch.

There are several reasons I didn't implement the support for mirror in cpanm, one of which was mirror needs to be able to group the requirements. (like with recent bundler).

mirror 'http://darkpan/' => sub { 
   requires ...
};

Or

mirror 'http://darkpan/';
requires ...

mirror 'http://www.cpan.org/';
requires ...

[zebardy]

I see, the idea of source groups in gemfiles. Seems kind of neat. I guess the first step for that would be working out the syntax or structure for cpanfile and how Module::CPANfile would expose this. I'd also be curious about how this might support defining multiple mirrors for a grouping (probably a small edge case). However I would argue that mirror does not need to support the ability to group requirements, at least not at first. That the slice of functionality of supporting global mirrors in cpanfile, cpanm and carton is a worthwhile one.

While working on this patch I started down the path of considering the dependency specific mirror lists, which are inherited by it's dependencies reading cpanfiles if present in dependencies and merging them into it's mirror list. I think this would apply to scenarios such as this. However thats, getting way more ahead of ourselves and far more complicated than is strictly needed for the main use case.