CPAN::Meta::Converter needs to support converting metadata fragments

[dagolden]

Per #40 we need a way of converting fragments as might be given by meta_merge fields into a specific version.

Something like this:

$cmc->convert_fragment( from_version => "1.4", to_version => "2" );

This could pick off pieces of the conversion specs. We need the "from_version" unlike the "convert" method because a fragment won't necessarily have a meta-spec field. (Though we could mandate one, I suppose.)

Alternatively, this could be done via an option to "convert", in which case, we do need to mandate a meta-spec field, and the conversion needs to use spec fields that match the input fields.

[Leont]

I suspect the ideal would be to have a default_from_version argument or some such. Being able to explicitly give the meta-spec is kind of required for this to be workable but different tool may have different sensibilities on what'd be the best default.

[dagolden]

CMC is per-hash:

$cmc = CPAN::Meta::Converter->new( $struct )

So I suppose we could let that take options:

$cmc = CPAN::Meta::Converter->new( $struct, { version => "1.4" } );

And then:

$cmc->convert( version => "2" )

Possibly, new should blow up if the provided version conflicts with meta-spec.

[dagolden]

FWIW, internally, CMC already has a 'spec' field, so that's probably the way to go:

$cmc = CPAN::Meta::Converter->new( $struct, { spec => "1.4"} );

Though we probably still need some way to not generate new fields, so maybe this:

$cmc = CPAN::Meta::Converter->new( $struct, { spec => "1.4", fragment => 1} );

[karenetheridge]

Is this totally done now?

[xdg]

This is done with CPAN::Meta::Merge.